Airsoft units are unsuitable for a number of reasons:
1. They're electric, as such, they will be rendered useless after the first sink.
2. They're too big, there wouldn't be enough space in the turrets of even the largest battleships to fit three of these and barrels.
3. They're electric. Must be said again, as whilst servos and motors can be scotchkoted, the mechanism inside the unit is very susceptible to water.
I know of the units you mean, because I own an rc tank with an airsoft unit, myself.
The weapons used are purpose built using a paintball CO2/HPA tank, with a series of pressure regulators to drop the pressure down to 100psi, where its fed into a copper/pvc accumulator of between 1 and 2 cubic inches, where a solenoid or servo-operated poppet valve releases that stored co2/hpa up into the barrel, where it picks up the bb/steel shot and propels it out the barrel at between 50(for the 6mm, largest calibre) and.. I'm not actually sure what velocity the smallest bb sized shot is fired at, but it still has the same muzzle energy of 1 joule. I could do the calculation, but I don't know the weight of the bb sized steel shot.
Must be steel, lead would risk water polution.
Any further questions?