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?
There is no reason to build it at the bottom it would be equally effective build anywhere underwater
OP specifically used the words “the bottom of the sea” and “the bottom of the ocean”.
And I stand by my comment that dikes are superior.