About soaking the battery in salt water, I can provide a bit of personal experience. We had a timelapse camera deployed in a BlueROV enclosure and powered by a BlueROV battery. Because we were in a rush, we only did a vaccuum test (successful) before deploying the system for 5 days at 40 m depth.
apparently there was a small leak, that produced a shortcut (records stopped after 6-7 hours).
When I recovered it 5 days later, water was filling half of the enclosure, was milky white (I guessed due to the battery leaking) and the internal pressure was enough to bend the metallic parts securing the the enclosure closed (and on the way up, with the water pressure decreasing the metal parts gave up and the enclosure “popped open”).
All this to say that is you plan to soak LiPo batteries in salt water, expect some degassing and some leaks in the saltwater (and thus appropriate disposal).
Epilogue: we now received the fixed timelapse camera and we will start very progressively testing it (dry lab first > freshwater tank > shallow sea short time > long time > deep short > real deployment)