I've been in June most visits and it's been busy but not too bad, just recommend visiting the parks early, or later on
Although if you do intend to stay for the day, don't just bail out the park during the afternoon rain shower, because from experience, it's so much emptier after it as a lot of people decide its hometime!
4th July in the MK was mobbed, we've done it once, made an ADR to ensure we got in haha! The CMs had to make a walkway through backstage to get guests up to the castle for the fireworks
We've found that Epcot's a good place if you want to avoid the madness on July 4th

we hit the American pavillion and met Mickey, Minnie, Chip and Dale, Goofy and Donald all dressed in their Independence Day outfits, and we were the only people there!
Although if you want crazy, go at Christmas time
Weather-wise, yes it's hot and humid, it is Florida after all!
Take plenty of water (i personally don't like water, so i take a bottle of squash with me so i can make it up)
Wear comfortable clothes, nothing worse than being uncomfortable, and uncomfortable in heat is even worse!
I don't find the squirt bottle/fan combo to be all that great, its just something extra to carry about with you!
If you're staying out for the whole day, try and hit the indoor attractions, or attractions with indoor or with air-con, queueing areas when the day's hottest (roughly 11.30am-3pm) If you're too hot in a queue then you're just gonna get grumpy, so hit the stores or go grab some food
Just be sensible about what attractions you hit at what times, make use of fastpass and EMH, hit the big queued attractions when they're going to be less busy (ie, during parade times)
Hope that helped
