It's a bit like if you're pushing a boat through a river and measuring something about the surface - you don't want to measure the ripples from the boat.
All sailboats need a keel of board for this reason: the narrow, flat surface of the board or keel produces little drag when the boat moves forward but resists motion sideways.
As I gradually master my buoyancy and hover 15 feet below the surface, I can see schools of small yellow groupers beneath the dive boat giving wide berth to a blade of a barracuda.