It's probably easier to bring three oceans worth of comets to rain on Earth than it would be to dig that deep and extract that water.

Eventually we will opt to dismantle the Earth to build more habitats, but it'll be more like peeling an onion layer by layer. By the time we are peeling 700 km, we'll encounter the water.

Who? Fundies assert that the world won't be flooded again (as per God's promise after the original flood). They are waiting for the world to be destroyed the second time by fire, not water.