For a sub, a ballast system would be needed, and if I were lucky enough to be working on such a project, I'd also need to be working on a solution.
I dont think that if i make a traditional ROV that i would even need a ballast. If the ROV has some weights on the bottem and the foam on the top and i work it a bit then i can achieve neutral buoyancy and have some more balance. and if there is a prop on the top or bottem that would make it go up and down. The lack of PWM with relays is a problem with me however. being able to pivot motors for steering in an ROV would be difficult and impractical. Lets say there are 2 motors on the back and one is put in reverse and the other forward. the turning radius would be nil. it would also make it easy to control with a hacked old joystick. and make a pc interface easy.
would i be able to make a pseudo pwm? where say if i wanted to half the speed then it would kill the motors every seccond half seccond? its obviously not the most efficent way to do it but is there any other way to pwm it? these guys must of done it http://binarybridge.k3.mah.se/