I agree that Obama was attempting to say that the capability of the Navy, based on the firepower of modern ships, was what was important, not the number of ships. He did it clumsily; it was not the best line of the night. If it had been, Romney folks wouldn't be able to use it like they are. If he had just said what he meant instead of trying to demean Romney, his point would have been clear.
That said, my takeaway is that, based on the capability of modern ships, the Navy has come up with a number (and not just a number, but numbers of different kinds of ships) that is needed, and we are cutting the fleet below that number. What I didn't hear from Obama was any thoughts about the "two war" capability. Still a Romney win for me.