Well, you can't build a ship in the water because, well, it just wouldn't work. However, large-scale construction in space is already a feasible concept in this day and age. Blast up the components and go for it. The only way constructing the ENTIRE ship on the ground would make sense is if there were some anti-gravity device on the ship that compensates for its mass and especially it's lack of an aerodynamic shape. (Kinda like how some non-aerodynamic combat planes today are kept in the air by computers.) Perhaps that's what shuttlecraft have, or Klingon birds of prey, or it's how Voyager was able to land, and it's why the saucer of Enterprise-D didn't drop like a rock. Maybe that's even what the mothership had in "Close Encounters". There'd have to be some sort of technology to prevent the ship from experiencing the loads placed on it by gravity, even while being constructed. Loads it won't experience in space. Are we to believe the neck on the Enterprise is strong enough to support the saucer's weight in 1 g, and the struts can support the engines? If so, since it's going to spend its life in space, that would be overengineering it in my book. Not building in space can be explained away. But building in space needs far less rationalization, and in-universe it seems a far more efficient method of assembly in the long run.