The ROV circuitry features a custom motor-controller setup, which is currently undergoing some serious rework. You can find the currently used schematic and PCB-layout here.
The motor controller is based on relays, in order to keep the electrical system as easy to understand as possible. The major disadvantage with this choice is that we only have ON/OFF control of the DC motors. Which makes the control less smooth. This is however a feature which is perfect as a DIY modification to the ROV.
The circuitry design is done using KiCAD - a free design tool for the job.