"It is true that you should break the water before juming in"

but WHY?

It's called surface tension. It's the same force that makes liquids form drops, allows water striders to walk on water, and makes thrown stones skip. It's at its maximum when the surface of a liquid is still and unbroken. Under those conditions, it sort of transforms the surface of the liquid into something akin to an elastic membrane. It increases the surface's resistance enough that someone moving with enough velocity (say after diving from a 100 foot cliff) may encounter enough resistance upon impact to cause injury. If it were to cause loss of consciousness the person would probably drown.