Stu, of the battery chemistries, Sealed lead acid batteries are the worst for the application you are using it for. SLA's are the heaviest, lowest tolerance for deep discharges, slowest charging batteries of the lot. They take 5 hours to charge properly, and cannot handle the high current discharges of RC electric motors.

On an airboat, you want your weight to be the lowest possible.

Lithiums are the lightest batteries per charge density, but require special chargers and special care to be safe. They require no faster than 1 hour charge, but can be charged slower. For your application you would need a 3 cell pack which would be listed as 11.1 volts. Their discharge capacities are limited to 20 times their capacity to date. A 2000mah (2Amp) pack can be discharged at 40 Amps. Of the battery chemistries lithiums are the most expensive.

Most cars and boats are run on nickel packs. Either Nickel cadmium (Nicd) or Nickle Metal Halide (NiMh). NiMh has the best charge density of the two but Nicad is the most robust. If you need 12 volts you can get that by using two series 6 cell packs. actually 14.4 volts. Peak chargers for these are cheaper than for the lithium, but can still have shorter charge times.

Most Brushless motor controllers have a Battery Eliminator Circuit built in to power the receiver.

For more information on batteries try, www.batteryclinic.com or www.batteryuniversity.com