Water jets aren't much use for cutting stone, unless you're attached to something very heavy. Otherwise, the stone is undamaged because you are propelled away. And again, you are doing this underwater, where the rules of physics mean you have different effects for the same event. Jet of water in air? Air is compressed until it moves out of the way. Jet of water underwater? Water doesn't compress, so you are stuck with it acting like a wall until it moves--in either case, the density saps your momentum.

Stone work all requires banging rock on rock. Coral is fairly soft, so it isn't useful for much. And again, much of the useful stuff is buried in silt.

Octopi are not deep ocean creatures, but rather natives of coastal shallows. They aren't doing anything below about 250 meters and do not inhabit the pelagic zones (that is, most of the ocean). Any plan that doesn't factor that in is unworkable.

Submerged waterwheels need some rather specific engineering to get around the issues of density and compressibility and are in fact much more akin to jet turbines than anything else. And parking one in a current just gets it torn apart since the current will be acting on both angles of the wheel simultaneously. Even if you do get one to spin around, all you have is a spinner unless you have already invented other machines to use that power immediately (mechanical energy doesn't store easily, and it's a lot of effort to get it to store in the first place--again, wasted energy is lethal).

And oceanic currents are created by the planet's orbit, not by temperature differentials. So is wind, for the most part.