I used a 5000 mAh LiPo from Hobby King. Be aware of a few things:
This will slow the speed of an already fairly slow boat. I highly recommend swapping the prop for the power one from LoyalHanna Dockyard.
You WILL need to reset the cutoff voltage, as 3S Lipo only starts at 11.1 volts, and needs to be cut off at 9 volts min.
You will also need to add a pile of weight in the bottom tray, as the LiPo only weighs half of what the SLA battery does.
The Lipo is slightly smaller that the stock SLA, so you'll need to make shims to hold it in place. I used pink foam.
You'll also need to use a LiPo charger, and preferably balance the cells.
That all said, it doubled my run time.