To float at the water’s surface or just below it needs only enough bouyancy not to sink, a few pounds will do nicely in calm water.
76 tons is the mass of the water an “empty” 40 footer can hold. Its actual mass depends on what is in the container and the weight of the container structure itself. If it’s filled with ping pong balls it will float with some degree of freeboard, if it’s filled with anvils it will sink, worst case for vessels running at or just below the surface a neutral or slightly bouyant box might be at, above, or just below the surface.
Bouyancy only determines where it is relative to the sea surface, mass determines how much damage it might do when hit by a vessel.