If it is based solely on mass, then I don't think so. A submerged submarine is almost neutrally buoyant[0]. It has the same average density as the water aroun

If it is based solely on mass, then I don't think so. A submerged submarine is almost neutrally buoyant[0]. It has the same average density as the water around it. Therefore, the mass of the ship is the same as the mass of the water it displaces, and a gravimeter wouldn't observe anything different. [0] Err, almost. There is no such thing as perfectly neutral buoyancy. In practice, they are very slightly heavy and fly though the water as a lifting body.