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?
Argh.
Look, imagine you have an aquarium. You fill the bottom half up with the solid of your choice. Gravel, sand, clay, whatever. Then you fill up the rest of it with water. You’ve created a simple model of the ocean, including the groundwater you’re talking about
Now dig out a hole in the middle and toss the sand you dug out into the trash. Your water level will lower, because you’ve removed some mass that was occupying some volume and the water will fill that empty space. You’ve successfully lowered the ocean level. It doesn’t matter how much groundwater there is, it’s totally irrelevant to the question OP posted.