MIB.
depends what power you require, twin waterjets eh. ?
Either will be power hungry, but if you look at the specs on the motor box it should tell you what power the motor should produce on each voltage.
I run a 900 BB Turbo single waterjet in a scratch built 1/16 PBR....and its ballistic !! mind you i run it on 14.4 volts!!!!!!!!! ( but its a 12v model ).
With water jets its all about torque rather than motor speed, so its what water the motor can force through the jet without slowing down and hence overheating...make sure you have some really good speedos in there, mine is rated at a constant 35 amps with a stall of over 75 Amps, the batteries tend to get a bit toasty,( good for keeping your hands warm on a cold day at the pond though.)
Sub.