Whatever breeze that may come off that lake isn't going to be enough to offset the fact that WDW in August is going to HOT and MUGGY!
You will feel some relief at night when the sun goes down but the humidity will feel like an invisible brick wall that you have to try and go through. Being at the Poly will help with transportation when you have had enough of walking in MK during the day and the Poly will be a welcome, peaceful sight.
Sitting by the pool in the middle of the day with the hot Florida sun beating down on you, will be like hard labor, unless you're in the water. Swimming at night and sitting by the pool then, is your best bet. Enjoy your stay.