I filled my professor's gas tank with water
That is terrible! Why would you fill his tank with water?
Because his lectures are terrible, he's always late and I'm tired of him
It is not good to fill up a gas tank with water, you could cause an accident or cause the car to be damaged. You will then have to pay for the damage. You can always drop the class instead.
What is the big deal? I'll just quickly pour it out when he comes back.
The damage will have already been done. You shouldn't fill your professor's car with water because you might be responsible for an accident. There are other ways to avoid him if you do not like him.