To be honest, it's getting intensely hot sooner and sooner. I started avoiding the parks in May last year and didn't come back until after Thanksgiving. I'd rather deal with rain than hot and humidity. You can always be prepared with umbrellas, or ponchos, or if you really do not care, or unsure of the rain, you can just make sure your electronics are in a tightly sealed plastic bag so when you're out and about and if you get wet at least your phone isn't. You can always find something indoors to do in the meantime if you're unprepared, or just don't wanna deal and wait it out. Also, rain stirs people away from the parks.
In terms of other than hot and humidity, the only thing to worry about would be hurricanes, not rain. Which as a Floridian if I was caught anywhere I'd rather it be WDW, it would be paradise compared the typical hurricane experience and safest to be, but for someone coming here for vacation it wouldn't be the best.
The few times I've gone out while hot and humid I used a cooling towel, had an insulated backpack of ice water and frozen water bottles, and use an umbrella to keep the sun off. When I felt my towel getting warm I'd adjust it to another area (as sometimes that area would just be getting used to it), or I'd pour more ice water on it. I typically didn't squeeze too much water out, as it helps make it last longer the wetter it is.