Everybody saying "wear whatever, it's so much fun that no one will care" are either blind, not human, under exaggerating, or lying. Yes, people do care. People dont care if your overweight, most people are. It's how you choose to dress. Some people are not made to wear a bikini, and they can't accept it. No one will ever give you looks for your weight, as often people are having too much fun to notice such things. However, if you wear something crazy, people will be sure to notice. Thongs (have seen it), speedos (gross), really hairy men who don't trim or shave (I gagged once), etc.. all gross at the water parks.
I have seen people go fully clothed to the water parks and it's quite funny. I have seen women wear a bikini with stilettos and chains and it was quite naughty, I have seen men who need either put a shirt on or learn how to trim that back hair. HECK, I have seen little girls run around with their tops off (I guess their parents forgot that Disney World is in AMERICA

) Th Disney water parks are unique because so many diffrent cultures are there, and because of this I as an American find some of the attire to be a little...bizarre. You'll be fine though, going there your either have fun where you won't notice, or when you do notice you will see that you look better than most of the people there, lol!!!
In your case, I think what you are wearing is fine. Imo, modestly is best. It's how I dress, and how I wish others would dress (at the water parks). No one needs to see everything. I'm a guy, and I was stressing over it, and I ended up going in board shorts and a smaller size shirt I had. Because it was smaller it looked like the wetsuit shirts, lol. I am so not someone who flaunts what I have, and honestly I wish others wouldn't either, lol.
Summery - DON'T STRESS!!! Put some thought into what your wearing and go towards the modest side of things (but not too modest). Even a little thought can make a big difference. Once you get there you'll have a ton of fun and not care at all. You're worrying too much, you'll have fun.