The world record for high dives into water is 177 ft / 54 meters by Oliver Favre in 1987.
Cliff divers in Acapulco routinely dive from heights of 147 feet 45 meters without injury. Most of the time. So highly trained divers who have practiced for years and know the water and conditions intimately can usually escape serious injury at that height.
However, the World High Diving Federation recommends that no one dive from higher than 65 feet / 20 meters without rescue scuba divers in the water due to the risk of injury. Once again, these are guidelines for skilled and experienced divers too.
Generally speaking, 50 ft / 15 meters is about the limit for normal people to not suffer very serious injury, and that requires that they hit right. You literally would bust a gut if you belly flopped from that height.
