Maybe I'm not grokking your question, but...
The force needed to raise the platform would be just its submerged weight: its dry weight minus the water weight it displaces. This is invariant with depth as the water density doesn't change significantly with depth.
The water column itself would have no weight at all, also regardless of depth: it is the same density as the water it displaces. (fresh vs. salt water aside for the moment.)
Think about it: a fishing sinker doesn't suddenly get heavier or lighter as it nears the surface. Its submerged weight stays the same. Why? Because the pressure force on a submerged object is distributed equally on the object, so the net force due to pressure is zero.