There was some drama about a bunch of Fisker EVs catching fire[1] after immersion, but apparently[2] not (directly) battery-related.

Offhand, I'm not sure if the specific battery chemistry requires vents for in/out-gassing, or whether you could reasonably fully seal them. Or I suppose you could[3] have the battery pack detect such a condition and use its power to electrolyse some water to keep the pack at positive pressure. Or maybe some kind of self-destruct that dumps the stored energy into a chemical reaction or passivates the cells somehow (wave hands)

[1] http://jalopnik.com/5956179/more-than-a-dozen-fisker-karma-h...

[2] http://www.motorauthority.com/news/1080277_fisker-determines...

[3] points will be awarded for the number of different ways this is a bad idea