With the default world generator, the floor in Deep Ocean biomes is all gravel with occasional pockets of dirt/sand/clay, but with the default TC config, the ocean seems to be more shallow and the floor has no gravel and is entirely made up of dirt/sand/clay.

This is a bit complicated to fix. In Minecraft, GroundBlock and SurfaceBlock are set to gravel and stone underwater, but to grass and dirt above the water level. TerrainControl does not currently support this.