Soft Water comes into play when a fall from any height at all can be rendered harmless or merely incapacitating if, at the end of the fall, the character meets a body of deep water (and sometimes, not even all that deep).

In the real world, falls into water from even moderate heights can be injurious if not done correctly, as anyone who's belly-flopped or back-smackered off a diving board can attest to. Falls from a sufficient height into water will be fatal regardless of how your manage the impact.