All lifejackets on board my boat are automatic, 6 150 newton Seagos and 2 275 newton Seagos. All automatic, all with integral harnesses, and all with crotch straps.
Never had one go off accidentally, although the child of one guest once pulled the inflation cord on one. Used them in pouring rain, in the dinghy
with boarding waves, wading in surf -- never a problem.
Automatic in case, as others have said, you go over and stun or hurt yourself as you do so. It can be a matter of life and death not to have to consciously pull the inflation cord, so I personally wouldn't have a manual one on board.
Ours have salt
pellets in the trigger heads. A splash won't set them off -- they have to be immersed for some time. And if one does go off accidentally -- so what? It's not a big deal to repack the unit and buy a new CO2 cylinder and trigger head (I keep spares on board).
One bit of advice: never, ever, ever buy a lifejacket without a crotch strap, and never wear one without fastening the crotch strap. Several deaths in the Fastnet 79 disaster occurred because of no crotch straps. You simply fall through the life jacket if nothing is holding you at the crotch.
In my opinion, lifejackets are defective and unfit for purpose without crotch straps.