My mother and I went durring the same time a few years ago. It was hot and humid, but there are plenty of ways to take breaks, all indoor shops, restaurants and indoor rides have A/C. But this is how my mom and I beat the heat:
We went to the parks the very first thing in the morning, oftern leaving the hotel by 7 AM. We would use Fast Passes whenever available. We would have lunch in the park between 11 AM to 1 PM depending on how we felt that day, and then went back to the hotel for naps and/or swims. We had dinner and went back to the parks around 6 until close.
The parks are crowed, but they are designed for this, with most ride lines being either indoors or in the shade, and Fast Pass is also very helpfull.
Always carry water when possible, wear sun-screen and a hat, wear light weight - light colored cotton clothes, and comfortable shoes, I can't stress the shoes enough!
Really, it's not as bad as it sounds, I don't think you should reschedule, and the water parks and pools are so much fun when it's hot! Just get to the water parks as soon as they open, sometime they fill up and they close the parks to additional people. Rare, but it can happen.
I hope you enjoy your time in the US, you're right, the 4th (as we call it!) is a great time to be here. I hope it cools down for you at home, we've been hearing about it on the news. It doesn't sound like much to us here, but we're used to it!