A second Torpedo Warship HT-2877 Motor Battery will let you run longer without having to recharge. This is a 6V 800 mAh NiCd battery, same as the stock battery that comes with the remote control warship.
It is advisable to let the boat motor cool down for approximately 15 minutes before installing the second battery and running again. The cooldown period will prevent the motor from overheating and burning out.