The RC boat ESC should move the thruster in the way that you expect. I am not sure about the PWM ranges that the boat ESCs use, so I cannot say if they will be compatible with ArduSub. If you show me a certain model, I might find out.
ArduSub expects 1100 microseconds to be full reverse, 1500 to be neutral, and 1900 microseconds to be full forward. This is how the ESCs in our store work, they are reversible.