I looked at the map before I read the description, so I was a bit baffled by what I was looking at.

The lines and dots you have representing the coast I'm more used to seeing representing the water, so that was an initial point of confusion. Depicting an underwater map is tough, but I think I might reverse the dots to the water side, and then include a few recognizable land features (beaches, cliffs, hills) outside the water area to help establish which part is water and which is land.

The only other suggesting I would make would be to add some variation the altitude/depths of the underwater cliffs to the depressed central area. Currently it looks very uniform, as if it had been artificially dug out.