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)