I dismissed the nanotechnology reference as having something to do with it's manufacture, not its operation. But you do have a point. "Nanotechnology" is one of those all-purpose buzz words that, by itself, means nothing, like "digital", "laser", or "quantum".

However, there is no reason why this technology need be limited by sheer mass. Instead of a 10-ton warhead with the power of 50 tons of explosive, how about 20 half-ton warheads each with the power of 2.5 tons of explosive. If this technology can be miniaturized so it fits on a conventional bomb or missile, it could certainly pose a threat to marine assets.

I also presume part of the effectiveness of this explosive is that it does not require the munition to carry its own oxidizer, that is provided by the atmosphere. In this case, it might well be incorporated into some kind of mine, one that pops out of the water when activated by a passing vessel.