I’m not proposing this as an actual solution, it’s just a dumb idea. But if we dug a huge, wide hole at the bottom of the ocean, or maybe widened the Mariana Trench or something, could that extra space make the sea levels drop enough to keep the land from flooding?
The big problem is that the bottom of the ocean is a very very very long way down.
Better to take the Dutch solution and build dikes.
There is no reason to build it at the bottom it would be equally effective build anywhere underwater