The accident occurred during an otherwise normal and intentional water landing in a flying boat. Due to crash forces, I was ejected through the windshield as the aircraft stopped in the water. I cannon speak to seatbelts, other than to say I was in the water outside the plane, but did not open my seatbelt to get there (I guess the going through the windshield part was an indicator of that too!). The aircraft remained afloat longer than expected, but sank as my fire department colleagues tried to tow it to shore with the fire boat. Happily, the accident was witnessed, so summoning emergency services was not delayed.
Smasher's comment is correct, never an automatic life jacket in a plane, and even caution with those in boats. You gotta be outside before it inflates. During my dunker course, I was required to "swim" front to back in the simulated deHavilland Beaver cabin while wearing an inflated lifejacket, and it was near impossible.