[Alex, The G and T]

Oh, this is my kind of question. One of my favorite pastimes is hurling myself off of precipices into deep water.

Well, a key point there is "deep."

however...

Speaking from experience... A ca. fifteen foot drop into into 5 or 6 feet of water: Land butt first, prone enough to increase drag, increasing proneness, on impact, guaging that fine line between a raspberry flop and too much slipstreaming. You will touch the bottom. (Do not enter feet, or head first.)

Less than five feet of water is going to get painful. I reckon that less than 4 feet of water yields a high probability of debilitating injury.

(All assuming a reasonably smooth bottom. An odd, misplaced stone will ruin your day.)