https://www.youtube.com/watch?v=ksKmBqRiUN8
If you're not planning on diving more than maybe a few times a year, perhaps only during summer, then you'll probably fine with just using rental gear. If the service and maintenance of such gear, especially regulators, has you worried about breathing through moldy equipment, it might be an idea to at the very least get yourself your own regulators.
As a vacation diver chances are you'll enjoy most of your time in warm waters, so 1st and 2nd stages with features for cold water are likely unnecessary. Although most regulators you'll find in your local dive shop are likely fine, the main thing to consider when picking one is comfort. This includes the comfort of the mouth piece (if you need to bite down a lot over time you'll quickly get jaw fatigue) and comfort of breathing. Many regulators allow you to adjust how easy it is for you to breathe, and this is likely to change as soon as you're underwater. Some have features like a two step free flow button, the ability to install the hose and adjusting knob on the other side if you like, or a mouth piece with its own inner part to bite down (see Apeks XTX50 as an example: https://www.youtube.com/watch?v=NhqArigpvfw).
Regardless which regulators and first stage you go for, it's recommended to maybe consider a different setup from, for example, the standard PADI one with a primary regulator and a long octopus for sharing air. Instead, consider getting a long hose which will serve as your primary reg. and also the reg. for donating air, and having a backup reg. right below your chin attached around your neck with a bungee cord which you should be able to put in your mouth without even using your hands. A panicked diver is likely going for the one you're breathing from anyway, and the long hose also makes donating air more comfortable for both parties. It's also likely quicker to offer the reg. that's already in your mouth in an S-drill (look it up) than trying to find your octo. There's nothing inherently wrong with the standard setup, however, so whichever you choose to go for is fine.
Although you don't dive often, another consideration might be a BCD and a weight belt. You should have no issue with whatever weight belt on offer for rental, but they tend to be fairly standard straps where the weights have a tendency to shift around in which can be uncomfortable. A better option is to get your own (plastic or metal buckle should largely perform the same) which is a bit thicker and more rubbery (higher friction material) to ensure weights stay in their place during a dive. You might also consider getting pockets you can attach somewhere around the shoulder of your BCD to have weights in to improve trim. As for a BCD, you're unlikely to go wrong with rental. They're all quite similar jacket styles with pockets, D-rings, exhaust valves (top and bottom), etc. It probably won't take up too much weight/space in your suitcase when traveling, and it might be nice to have a BCD you know you're comfortable with.
You could also consider a BPW. You're likely to get better trim with it, there's far less unnecessary features like you have on your standard BCD, it's more extensible (so you could make it BCD like), and it's likely to take up less space in your baggage since it's either a more sparse jacket style like BCD or just a harness, bladder, and a backplate. You can also go for a light-weight backplate to save yourself more weight. However, this might be going a bit far for your average vacation diver.
A dive computer is another consideration. Depending on what you go for, you might be able to transfer dive log data to your PC so you can log your dives on something like Subsurface, where you might be able to see your gas consumption (if you have a pod and compatible computer), tissue saturation, etc, which is neat. A dive computer won't take up much space either so it could be a worthwhile investment. A regular diver might should consider going for something which could serve you well should you ever consider trying technical diving, like a Shearwater Perdix, Heinrichs Weikamp, or a Suunto EON Core (EON Steel goes to 150m unlike the Core's 80m).
A wetsuit (or semi-dry suit) can be a worthwhile investment if you find a really food fitting one. There will be less circulation in a good fitting wetsuit, which means better thermal performance.This is one piece of equipment which isn't easily adjustable after the fact, and it's better to know you're not swimming in something someone else has peed in (rental). A drysuit is another consideration since it adds a backup buoyancy device and actually keeps you dry, but will quickly add up to a lot of weight for travel, especially if you go for neoprene instead of a shellsuit. The latter would serve you well in technical diving, and you can pretty much use it in any climate. Some people are more comfortable with wetsuits though.
If you're a regular diver, why rent anything? Bring your own gear, and maybe half of it if you're going on vacation. A twinset could be a worthwhile investment, and will serve you well into technical diving if you have any plans of ever doing that. However, if people around you are mostly using a single cylinder, you might just end up just carrying extra air for nothing (unless you use a lot of air). Twinsets also requires backplate mounting, which means less compatibility with single cylinders if you were to take your BPW with you on vacation.
One worthwhile investment is in a longhose and neckreg setup (DIR setup), regardless whether you go with a twinset (two first stages) or a single cylinder. See equipment guide.
https://www.youtube.com/watch?v=E45OLt5hlIg
During the warmer months most all of the FL springs are packed. Late Fall/Winter is when the crowds die down.
There are a few good springs that allow open water divers. To list a few, Devils Den, Blue Grotto, Vortex, Paradise, Buford. While they allow open water divers they are still overhead environments for the most part. Dive within your limits.
As far as ocean the choices are vast. I don't do a ton of ocean diving but to name a few good charters, Tanks A Lot out of Tampa, Horizon Divers out of Key Largo, and in West Palm/Jupiter area, Pura Vida, Jupiter Dive Center, Stuart Scuba, Pirates Paradise.
"Milwaukee area, call Shipwreck Explorers or Double Action Dive Charters. The Len-Der is ok but doesn't post their calendar anymore. The car ferry is one of my favorites, the prins William is fun, and the dredge is cool too." - Anon. >>>/out/1996031
"El Pit Cenote in Tulum"
"was pretty epic, found pirate treasure behind the stop sign!" - Anon. >>>/out/2031612
"Brian Kakuk is a well regarded cave diving instructor and the Bahamas have beautiful cave systems." - Anon, paraphrased. >>>/out/1920859
Wayward Wall, Jost Van Dyke. "It's a nice, easy and peaceful dive with a max depth of about 65ft and zero current." - Anon.
Pretty nice dives around the area. Shaab Claude divesite is recommended. From Anon. >>>/out/1920269
"amazing night dives in Eilat, but its basically the less awesome version of Sinai cause its the tip of the red sea and its a tourist city so the reef is pretty beat up there." - Anon, probably talking about the Gulf of Aqaba (which includes the well-known Dahab).
"in the Andaman Islands(specifically from Nicobar island) you used to have dives that went out to Barren Island, one the most pristine diving sites I've had the privilege of seeing, I'm talking field's of corals going out around as far as you can see with a sheer rock wall on your one side with 60-50 meters of clarity." - Anon
Controversial for producing a mixed bag of divers and dive instructors. Apparently a great place if you're an instructor who needs to get experience under his/her belt.
"Koh Tao specifically is the place to be if you just want bulk work since its jam packed with tourists, you can get plenty of experience and make a humble pay that covers the basics(housing, food, booze, weed)." - Anon.
"went to iceland last summer and did a dive in the silfra and two at the strytan water in the silfra is incredibly clear, "trolls hair" algae look pretty cool, sadly no fishies :( lots of cool fishies at the striyan, best one is the wolfsfish"
Images
"second dive at the strytan was at the geothermal vent so really unique warm water coming out of it which is nice when youre in the cold water and cool fishies and jellyfish that light up in all colors when you point your flashlight at them" - Anon. >>>/out/2036350 >>>/out/2038329
Open Water diving skills: https://www.youtube.com/watch?v=ZRVktsXO9FI https://www.youtube.com/watch?v=q0VHnPLlvzg
What happens if you don't equalize: https://www.youtube.com/watch?v=Y2KJy5UBgiA
Reel and guideline use: https://www.youtube.com/watch?v=fQtcjyI3iuw
Backwards kick: https://www.youtube.com/watch?v=mYhnhx3v4dY
Cave diving: https://www.youtube.com/watch?v=kQyekjC5URY
Operational aspects of technical diving: https://www.youtube.com/watch?v=F_75zZSjj_Y
Respiratory failure in technical diving: https://www.youtube.com/watch?v=QBajM3xmOtc
Decompression controversies: https://www.youtube.com/watch?v=UY61E49lyos
Deep Stops Updates: https://www.youtube.com/watch?v=DjOx1kcIrTc
Alternate hose failure: https://www.youtube.com/watch?v=lR_PRmeUAyM
Discover Scuba Diving gone wrong: https://www.youtube.com/watch?v=dZjgEEAifYU
Weight belt accidentally dropped at 30m: https://www.youtube.com/watch?v=B5ZuLx6aA_U
Running out of air: https://www.youtube.com/watch?v=Geu_ZuhHOLM
Bad diving etiquette: https://www.youtube.com/watch?v=FRzyNUw-llQ
Regulator failure: https://www.youtube.com/watch?v=ob4aePYWRwQ
Diving accident and bad air sharing: https://www.youtube.com/watch?v=itHw6zgVigU
Diving beyond your limits (mute sound recommended): https://www.youtube.com/watch?v=W30cufYc_ZI
Out of air with nonchalant buddy: https://www.youtube.com/watch?v=43LHQN3ijrw
Out of air situation: https://www.youtube.com/watch?v=lN8dZh6ho1s
1st stage failure: https://www.youtube.com/watch?v=b-KCiky13bM
Signs of Stress (with commentary): https://www.youtube.com/watch?v=TGGyUwtfVtU
CO2 Panic: https://www.youtube.com/watch?v=nm65KECsEpE
Tryhard Safety Stop: https://www.youtube.com/watch?v=laf8225Lgkk'
Mouthpiece failure: https://www.youtube.com/watch?v=H6yMHXCCAUE
Broken low pressure hose: https://www.youtube.com/watch?v=Ho0eNR4jvEg
Slight panic during exercise: https://www.youtube.com/watch?v=0etRBn8zYm8
Accidental weight drop: https://www.youtube.com/watch?v=xnEuj-JKmqQ
Violently panicked diver: https://www.youtube.com/watch?v=SF3dQDgynT4
Caught in downcurrent: https://www.youtube.com/watch?v=rjSfcKpP6iU
Full panic bolt-to-the-surface diver, refusing regulator: https://www.youtube.com/watch?v=CFCVVhFbUuY
Out of air incident: https://www.youtube.com/watch?v=30Dh0W_cPpg
Another weight belt incident, but where camera is more important than life: https://www.youtube.com/watch?v=_O85LwqCB7k
Infamous Yuri Lipski dive video (dies): https://www.youtube.com/watch?v=cRj0lymMMGs
At one point someone might just drop an anchor on you: https://www.youtube.com/watch?v=4xSaus7ovHI
Hilariously bad dive with someone almost dying: https://www.youtube.com/watch?v=pNboqpaJuDY
Badly handled out of air incident. There should be its own video on YouTube without the bad Daily Mail take: https://www.youtube.com/watch?v=8Gc-254fQBY
Out of air with panicked ascent: https://www.youtube.com/watch?v=XujePQIafRM
Decision is made to go beyond limits and almost die: https://www.youtube.com/watch?v=fttvAKJPSd4
Suicide dive with first stage failure: https://www.youtube.com/watch?v=AqKUUrlK2tM
Failure to equalize mask resulting in mask squeeze: https://www.youtube.com/watch?v=u5nY58I284Y
It's a pretty based hobby if you like getting wet. Staying and breathing underwater for longer periods of time exposes you to a completely different world and experience. You should at least try this once in your lifetime. Also, there's no talking underwater (except occasional hand signals), so you won't have to deal with nagging from other people except for a short period on the surface.
Any hobby has its expenses. If you choose to own your own gear instead of renting it you'll quickly drive up quite a hefty cost. However, if you turn out to be an active diver the initial investment is usually worth it over time.
Yes, you could die with very little effort (hold your breath and ascend: boom, you're dead). You should keep this in mind every time you dive. The risk is worth it though, and quite low when it comes to recreational diving between or shallower than 18-20 meters.
Only if you accept x2 the risk of dying.
Get certified (beware Ko Tao in Thailand though). There are many organizations who certify divers, notable among them PADI, SSI, GUE, CMAS. PADI is probably the largest organization, but is usually a little more expensive than alternatives. Before even getting certified though practice only breathing through your mouth.
Besides likely getting water in your mask while diving at some point, there's mask flooding and mask off exercises in the course. You don't want to bolt to the surface in a panic because you suddenly snorted some water. For practice, try snorkeling with swimming goggles (which won't cover your nose). Don't forget about mask squeeze though (see video in guide).
The guide is sparse for now, but if you can't find anything there then ask ITT.
Subsurface is free as in freedom software for logging and planning dives, and it's quite possibly the best software out there. A physical logbook is also fine and should probably be used in conjunction with it.