A battery:doh
Not a yacker, just a PFF addict hear that reads most of the posts. So take what I say for what it's worth.
Most either have a small battery like the powerwheels batteries or they make a battery pack with AA batteries like this and put it in a waterproof container of some sort.
http://www.radioshack.com/sm-8-aa-ba...i-2062251.html