The energy required to lift that amount of rock from the seabed to above the surface would be impractical. But good news! It doesn’t need to be from the bottom of the sea, just a part below the desired waterline. So we can dig these big holes you want right off the coastline and then, as a bonus, use the materials as landfill to raise or extend the current coastline. Still wildly impractical, but much less so than digging at the bottom.