Bring a piece of carpet and put your drummer on it....... it'll help a little. The thicker the better...
Beware of splashers, try to keep the equipment awayyyyyyy from the pool and make sure it doesn't look like a convienent table for people to set drinks. Whatever you do keep the instruments in cases because people milling around can and will knock them over.
Sound is weird outside, you may not have as loud of a drummer as you think once you're outside. Have fun

Oh, and bring garbage bags/tarps to cover stuff in case it rains.