Is the density of a submerged submarine about the same as the density of a floating submarine?

The average density of a submerged submarine is about that of the water that surrounds it. It is said to have neutral buoyancy. But the average density of a floating submarine is less than the density of water, otherwise it would not float. (It could be argued that the density of a submarine is constant and that its characteristics as regards buoyancy are manipulated.)

A submarine has "empty spaces" inside the pressure hull. That's where the equipment that makes a submarine work is (along with the crew). When a boat (the submarine sailor's term for his vessel) is submerged, the whole volume of the vessel displaces a likevolume of water. This is true, except that the main ballast tanks are full of water; they have no air in them. The submarine is set up to be neutrally buoyant when it's under water. That way no "effort" or "energy" is spent trying to keep the boat from sinking or floating to the surface.

When a submarine is on the surface and the ballast tanks are filled with air, the whole boat floats. The extra air that has been put into the main ballast tanks has displaced the water that was in them when the boat was submerged. This extra buoyancy allows the boat to float and operate on the surface.