Larry & MossLandingCreatures:
Presumably, nobody will be disassembling their LiFePO4 battery cells while operating their boat.? Practical tests indicate that the main hazard with immersion in sea water is the potential for arcing across the terminals, and not necessarily water intrusion into the case where the moisture can interact with the Lithium.
The CALB SE200SI cells I'm using, for instance (prismatics, and not cylindrical cells) have a mfg specification requiring that they endure submersion in sea water for 2 hours without fire or explosion.? Some tests indicate the possible leakage of electrolyte in these circumstances, but no explosions or fires.? Fresh water immersion (for us Great Lakes sailors) poses a somewhat lower risk.
IMHO, a cover over whatever box you're containing (and restraining!) your cells in, ought to avoid any issues except where you've holed your boat and you're literally sinking, or you've rolled your vessel and flooded the mechanical compartment.
It doesn't seem practical to protect against EVERY possible negative scenario.
[-tv]
THOMAS VANDERMEULEN