Sudden Death ! ! ! What Happened ? ? ?

I was drifting in a boat about 5 meters off shore . . .
In a 2 block deep area of water . . .
Looking at something on the Shore when BAM ! ! ! - I was DEAD ! ! !
Respawn screen shows me with all under water bubbles . . .
All I heard was a SPLOOSH sound . . .
2 Seconds later I was looking at the respawn screen . . .
What in heck Happened ?????
What KILLED Me ?????

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  I Also spawned 3 nether portals in this same game with VERY little or Nothing to step out on with a lava ocean right below . . .
  Roasty Toasty . . . Go Figure . . .