Yes, August is very, very hot. Personally, I think September is worse, but August is definitely hot.
Will you regret it? Nah. Just go prepared. Have water with you at all times. Not soda, not juice; water. Go through the air-conditioned stores whenever you can to avoid the heat. Enjoy those water rides and wear clothes appropriate for it (not denim shorts, for instance). I go in mid-September, and the crowds are non-existent at that time. If the crowds are low at the end of August, plan on park touring in the mornings and late afternoons, and hit your townhouse during the hot hours to "recharge" and freshen up.
Bring plenty of sunscreen, wear a hat and wear light colors. Be prepared for nasty afternoon thunderstorms. They're over quickly, but you don't want to be outside for one -- lots and lots of lightning.
No, you won't melt, but you will sweat ... a lot. There's no way I can do a park all day and then head straight out to a nice dinner or strolling in Citywalk or DTD at that time of year. I'm icky. Must head back to accommodations to freshen up usually in the early afternoon, then head back out.
Honestly, I really can't stress the water enough. Dehydration will ruin a vacation pretty quick, and it happens fast in Florida's hottest and most humid months. We consume about one 12-oz bottle of water an hour while in the parks; sometimes even more if we're drinking alcohol, too. Don't wait until you're thirsty. It's too late then!
Have fun
