Also the automatic lifejacket has been known to go off accidentally when it's a big wave and really wet,
I think Hamish wrote a bit about auto lifejackets on here once. He knows a wee bit about them
The mechanics of a modern auto system means that it is extremely unlikely to go off in heavy rain, even a monsoon and water spray. Water has to dissolve a "plug" before the gas is operated. That plug is protected from rain etc by being tucked up inside. It needs submersion for a few seconds to work. There is still a toggle incase it does not go off.
Manual is great if you are conscious, not panicking and can find the toggle
If Hamish could put up a link to the correct use and wearing of Lifejackets that would be great !