If you place the empty lot, then using the cheats, type in
testingcheatsenabled true
enablelotlocking true
then change the lot to a dive lot. Go into the empty lot, and press the flatten lot tool, and it will sink it to the bottom of the ocean. Return to edit town and change it to residential and place your lot.
However,
You won't be able to move your sim straight in there, even with the freerealestate cheat.
When you move your sim to the house, they will teleport to the front door area, but then not be able to access anything in the house
You won't be able to move the camera under the water.
So in short, you can do it, but it won't work.
