Sounds a great idea in principle, but might be more complicated than at first thought.
Not an expert on the physics here. Something along the lines that sea water has a freezing point below zero, and there is an oddity where the fresher part of the water freezes first, making the remaining water progressively more saline. So there is not a single temperature where the water freezes. The saltiest bit freezes last. Possibly at minus quite a bit. -20?
I guess in reverse the saltiest bit would also defrost first.