Hi Hans -
Is the GPS you seek to be used when the ROV is at the surface? Cerulean sonar has this unit, but it does not include an IMU (inertial measurement unit aka. compass.) It would give you course over ground if moving, but not much help when stationary.
As for using an external USB motion sensor … while you may be able to find these on Amazon or Tindie, getting them integrated into Ardusub may require customizing and compiling your own version to support the hardware. This is a tricky and involved process, and you don’t have the guarantee that it will eliminate your interference issue!
Therefore, I’d strongly recommend seeing what you can do in your design to avoid any wires carrying high currents from passing close the Pixhawk or Navigator you may be using.
It sounds like you have quite an interesting project! Remember that when using multiple ping sonars, you will need to time their pulses so they don’t interfere with each other. Good luck!