ROV - underwater with APM?

Would an APM 2.x work in an underwater ROV, obviously with no GPS signal?

Will the APM Mission Planner be able to track movement based on a compass and gyro/accel data alone?
Here is my plan for the h/w from the ROV:
APM - USB to RS485 converter - CAT6 cable, 500’ - RS485 to USB - Laptop w/Mission Planner
also on the CAT6 cable I’ll have:
Camera Composite- Balun - CAT6 - Balun - Composite -Video to USB Converter - Laptop

I will also have sensors for temp and press going into the APM, and will have a couple of digital outputs for light control. I’ll be driving 6 ESCs, with a LiPo battery in the ROV (i.e. no power over Ethernet). The way Mission Planner records and displays everything would make it ideal for an ROV.
Any thoughts would be appreciated. Thx

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