• Power 2 to 4 DC motors 4.5V to 36V at 0.6A each
0.6A is not a lot of power, my RC Cars can briefly draw 100's of amps when accelerating, I understand that boats are even more demanding.
These guys use Arduino in their work and I am pretty sure I saw that they were working on a submersible of some sort, it might be worth having a look around their site for ideas, I have seen them on youtube driving/sitting on a mobile phone controlled boat that I am sure was driven by Arduino.
illutron.dk
Duane B