There are a couple things wrong with this statement, actually, there's a lot of things wrong with this statement. Plenty of things can go wrong when you do MDMA, and staying hydrated won't prevent all of these things from happening.
First off, there is such thing as over hydration, and it's actually more likely to be a problem when you're on MDMA (google ' MDMA related hyponatremia' or look it up on Bluelight) So you actually shouldn't be drinking too much water.
Secondly, since you've never taken MDMA before, you don't know how it will affect you, and not everyone has an awesome time on it. Being at a rave can make some of these things worse. MDMA has been known to cause seizures in some people (seen it happen, NOT FUN) and can just not be fun for others.
Finally, there's more than just your health to worry about. Cops can be a problem once you leave the rave, and since you're underage, so can your parents. Rolls last between two and ten hours depending on the person, and since it's your first time, you're going to need somewhere to go where you can be fucked up without any problems after the rave, or during because you may not enjoy being high at the rave and have to leave early.
Don't go on the logic of "I heard rolling lasts three hours, and we'll be at the rave for four, so we can just go home after" not only will you likely be high for longer than you'll be at the rave, but you're pupils will be noticeably large until you go to sleep (some people still have saucer pupils the next day) So plan on having a place to go where you can be fucked up without any problems, and a safe way of getting there. Be smart, have a sober driver, and if you can't get one DO NOT TAKE THE BUS OR ANOTHER FORM OF PUBLIC TRANSPORTATION. It's very likely someone will notice you're on something and won't like it, don't call a parent for a ride either. (lol) If you don't have a sober driver, call a cab. It's a bit more expensive, but it's worth it in order to not have any problems.
I'm not trying to scare you, and don't worry too much, just keep these things in the back of your mind and plan ahead to avoid any problems. Also, if one of your friends is having a bad come up/roll at the rave, just leave with them. It's better to have all of you leave together than have some stay and some leave. You guys will have plenty of fun at home or the rave, so you might as well stick together if one of you ends up having to leave.
Be safe, and have fun!