I don't want to go on my friends wedding cruise
What is your reason for not wanting to go on this cruise?
Because I'm not interested in going to a cruise and I'll be there just  for the wedding.
Tell your friend about how you feel. Be there for the wedding but not for the cruise. I am sure your friend will understand.
What if they get mad and thinks like I'm evading them.
Do your best to explain to your friend why you do not want to go on the cruise but just the wedding. If you get sick on cruises or simply cannot afford it than it makes perfect sense. Either way, i am sure your friend will understand and value your honesty!