The bug

Soul sand and magma blocks can either make an entity float to the surface or sink in water, respectively. But once placed, they will have a 1-2 second delay until they start functioning.

Also, when placed in flowing water, they don't function at all. This means underwater secret ejection systems will be much harder, so will concealed underwater whirlpools, as when the piston door opens, the soul sand or the magma blocks will be covered in flowing water, and will not function.