You also need something that allows the Arduino to control the high currents required by the motor - usually a h-bridge or an ESC. An ESC designed for radio control might be the easiest option as it is controlled like another servo.
It is easy to imagine how to control your boat to move and steer.
However you also propose to use sonar devices and an accelerometer and you have not explained what you want them to do.
Have you a link to the specifications for the sonar board?
Do you mean there will be 4 sonar management boards or that one board will manage 4 sonars?
Have you links to the specifications for the sonar devices and the accelerometer?
...R