We have gone the last three Augusts.....2012, 2013 and 2014.
Its very, very hot (this past year in particular, the temperatures were incredibly high) and very, very humid.
For August visits, we primarily focus on the waterparks, the beach, pool time, and evening activities (mini golf, etc.). We honestly don't know how people do the Disney parks from sun-up to sun down, day after day, when its 100+ in the shade.
That said, we have very much enjoyed August (obviously....we went back twice more after our initial experiment!!
) and have had some wonderful vacations then. We must have been very lucky, because we didn't find the rain problematic on any of our trips (one week each in '12 and '13, and a full two weeks in 2014.....this past year, it literally only rained 4 times in 14 days). That said, when it rains....it RAINS. Harder than anything we see here in the north. So when you do get wet, you're generally gonna get soaked.
The heat takes a lot of of a person, so you have to be prepared for that too. Its taxing.
Drinking buckets of water is essential. Dehydration can be a real concern. We literally drank a dozen water bottles apiece on our SeaWorld days last August.....thank the lord for those inclusions on the all-day dining plan.
Orlando has some fantastic waterparks. Aquatica, Typhoon Lagoon, and Blizzard Beach are all great choices for summer visits. There's also Wet 'n Wild (but I haven't personally given that park a try just yet). You would definitely want to incorporate at least a couple of those in your plans. The beaches are also beautiful and surprisingly empty at the end of August.....we have done both Cocoa Beach and Daytona Beach, and would recommend either. There's nothing like a day at the ocean
.