That radar is actually measuring the sea height. Sea height (on average) more-or-less defines the geoid (the height of an equipotential surface) over the oceans

That radar is actually measuring the sea height. Sea height (on average) more-or-less defines the geoid (the height of an equipotential surface) over the oceans. That ignores sea height disturbances due to winds, currents, tides, etc. The geoid, in turn, is sensitive to local mass distributions. So yes, "it has been done", but the result is nowhere near the resolution or interpretability of a sonar survey. Still, it would greatly surprise me if the military _wasn't_ already using the technique. So, if there were interpretable results from the technique, I would personally expect the Navy already had them...