WW2 Navy Video

Zitat von Raptor1

The Type 23 grenade, originally a Chinese design, did not have this feature. The Japanese also developed the rather crude Type 4 grenade later in the war but I am not sure how hard it was to activate or if it would have worked in water.

Also, it could have been a captured grenade from any number of enemies, I can see why a Japanese pilot would keep one in case something like this happened.

Right, since we don't have a clue what type of grenade that was it's a dead end. This also explains why some of the downed pilots where shot while approaching the boat. There was a real threat that grenades carried by these pilots could be tossed onboard a ship killing or wounding people there.