You can recycle the vast majority of the water used pretty efficiently, it wouldn't be a huge problem - There's other much bigger ones, such as the daft idea of building a spacecraft the size & shape of the movie Enterprise. If you are going to build a spacecraft to travel around the solar system you'd use good engineering, not movie fantasy. The only reasons the Enterprise looks like it does is just for the movies, you would never build something like that in reality as it wouldn't be stiff enough and is a very poor use of materials for the volume required.