I was playing around in a vanilla survival world (other than OptiFine), jumped off a mountain into water (I know for sure I hit the water, not a lily pad or ground). A slight, maybe half second delay occurred before the game said I died with a death message simply stating "death.fell.accident.water." Since I had the debug screen up, it clearly shows I was in the water, because the Y position was 62.12315! It should be noted that I had some lag at the time of my death, but there was no possible way I could've landed on land.