How deep can I go if jumping from 50m?
First one can get killed even by coming in contact (with speed from high altitude) with the water surface, which at this speed and momentum it appears as a "block of cement" (or more correctly, develop high enough forces to break your bones as per @dmckee's comment).
This depends what wil be the impact surface (that is why seals and olympic divers fall into water with a minimum surface of impact, i.e perfectly vertical).
Then the height (or depth) necessary is found by knowing the altitute of fall, plus the weight of the body. This enables to find momentum at time of impact with the water surface.
Momentum at time of impact plus the buoyancy factor of the water (which depends on amount of salt among others) gives the depth the body will reach. Then the safety depth is that depth by adding a safety margin (if you want an engineering approach and not just physics one)
Assuming you do survive the impact with the water, according to this answer all you need is 4m.