I usually am finding spot with shallow water extending quite a bit into the ocean, and rise a path with hoe as far as I want and build around it to stabilize the construction instead trying to reach the bottom floor.

Shallow water, but not shallow enough for spawned enemies (mostly dwarves) can't stand in it to hit the boat. And not to build too close to troll cave / swamp and other things that can easily destroy your way of transformation without you noticing while you're doing something in a base nearby.