Doing certain movements in water can cause fall damage that can sometimes instakill the player (I died to it while playing survival in 1.13-pre6). To reproduce, swim at least 6 blocks off the ocean floor, then swim to the ground at any downwards angle while sprint swimming and sneaking to sometimes take damage proportional to how far down you dived while in the described position. (Note: This doesn't always happen and only happens about 1/25 times and you may have to do it for 3 minutes because this only happens occasionally)
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If you want a better way to reproduce, teleport yourself at least 24 blocks above the ocean floor, then sprint swim once you hit the water and then land on the ocean floor to "fall out of the water".
It says "Player hit the ground too hard" now because the previous string "Player fell out of the water" was removed in 20w12a (MC-173487).
In the screenshot, I borrowed someone elses account, set difficulty to peaceful, turned off regeneration, and gave myself conduit power to reproduce the bug again. The floor was 7 blocks below me when I died, and the game seemed to lag a lot before I actually died from this glitch. There could have been nothing else that killed me.