Hi @StrikeLines,
Ouch, sorry to hear that, that has the potential to cause some serious damage. None of the components on the BlueROV2 have reverse polarity protection, so you’ll need to check just about everything.
The first set of components plugged directly into the power rails would be the first to check. This will include all the ESCs, PSM (power sense module), BEC (5 v converter), lights, and the ROV side Fathom-X board. Check those one by one if you can. After that, check the control components powered by the BEC, including the Raspberry Pi and Pixhawk, but perhaps just the BEC died and those are fine.
The thrusters should be undamaged.
-Adam