I'm curious why a starship should be compared to a conventional naval vessel for density. Is there some reason it needs to have a density of less than water? If we're going to talk about common sense, the material descriptions we're given about starships is that they're designed to withstand weapon impacts that destroys relatively dense things like iron-nickel asteroids with ease. Even assuming that say torpedo technology evolved significantly between 2293 and 2370, an unshielded Enterprise
-A was taking torpedo hits that would presumably destroyed much larger asteroids with relative ease. In which case the hull materials being less dense than a combination of nickel and iron doesn't make a lot of sense. Indeed the requirements for a starship constructed in space (contra the absurdity of ST09) are probably more toward better thermal properties and maybe radiation shielding over things like lightness. There's simply no need for a starship that uses mass reduction technology to worry all that much about using high density materials for its hull.
There's nothing in canon or even official sources that suggest a 190,000 ton Enterprise
, and there's evidence to suggest ships are, in general, heavier than water. There's no real reason for the compromises that naval vessels have to deal with in terms of hull density on a starship.