Mine have been out in fairly heavy spray (my skiff is a flat-bottomed Bristol 17), without firing, so I think that anything short of a constant heavy rain or a really severe wave dunking wouldn't trigger it. Still, as you point out, one may not be conscious as he hits the water, so I'd rather have an occasional misfire, than a manual that is undeployed because its wearer is unconscious- and dead.
The recharge kits run about $14 or $15. The manufacturer recommends testing and rearming the SOSpenders every other year, I think. It's a simple matter of changing the CO2 bottle and replacing the trigger, then repacking the inflatable. The manuals, of course, have no trigger, but you still have to change the bottle, and repack. One is packed inside each vest when you get it. Incidentally, both models have an air tube for emergency manual blow-up.