R. Star wrote:
Because mass is such a huge factor in the weightless environment of space.
Being "weightless" has no effect on mass. Water's mass is 800 times that of air. A cubic metre of air is 1.25 kilos (abut a loaf of bread), the same volume of water is a metric tonne.
Put the impulse engines of a runabout on the Enterprise D, and they will (eventually) move the ship around.
Two otherwise identical ships, one with air in it's internal space, the second with water. The one with water will be "loggy" in maneuvers, it's inertial dampeners will have to work hard, as will it's structural integrity field.