Here is what it looks like when only the SurfaceBlock air (SurfaceAndGroundControl: AIR,DIRT,0.0), it can be used to make some cool effects if the biome is exactly at sea level.

Here is what happens when both SurfaceBlock and GroundBlock are air (SurfaceAndGroundControl: AIR,AIR,0.0)

For 1.16.5 - 0.0.15, any sagc block that is air (after replaceblocks) is set to waterblock when between waterlevelmin/max.