With the new update for this wonderful addon im not aloud to build them inside float cubes, Please fix, and maybe its a float cube thing...
That is because isWater = 1
There are actually two building layers, the main layer and the "zone" layer. Water bricks, zonebricks, floatcubes, and (for now) these all build in layer 2.
I think Badspot, to help replace interiors/terrain in v21, should add in third and fourth layers.