I tested my inflatable life jacket last summer - by accident. I went out to the amphibian to go flying, and found my lifejacket sitting inflated on the seat.
It would seem that the water in the bilge of the fuselage, combined with a few very warm days, and a closed cockpit, made it humid enough to dissolve the little catridge which actuates the automatic deployment feature. I don't use the automatic feature in the plane, but had moved the lifejacket from the boat to the plane a few days earlier, and not disarmed it yet.
An easy fix with a rearm kit, $40, but now I change out the little yellow cartridge more often - they're much cheaper than gas cylinders!