the faster you fall, the harder you hit the water,the more the water acts like a solid, the faster it slows you down,so the less you need,it's not the fall that kills, it's the sudden stop at the end, if you go too high, you splatter your self on the surface of the water before you have a chance to go through it.
When i jumped off the top of Red Rock, and the Punchbowls i didn't go as deep as when i did the little 20 footers.
if you don't believe me, do some big jumps and find out for yourself
When you land flat on your back, from even 30 feet, you sink into the water slowly. If it wasn't for the splash you could almost keep your hair dry. Does anyone really want to do that? You need at least 10 feet of water, 8 if you are a total lunatic.
well ya i've done the sand jumping thing at a party. 15ft or so, lol the ladies love it. and i've jumped off a train trestle into about 7ft of water and it was around 15ft up.
i do the waiheke one (18m, which i think is about 60ft?) at low tide into about 8 foot of water.
that is because i know that it is a sand bottom, and hitting the bottom feels like jumping off of a picnic table onto the ground.
so hitting the bottom is not a problem, as long as you know exactly what is down there.