You can never get all the water out. Hoses point up/down and sideways. The waterblocks and radiator also have nooks and crannies. Unless you were to mount it all on a huge piece of wood in a linear fashion, you can't get the water out.
And filling a loop isn't as easy as pour it in and go, done, it usually takes a few hours to fill and bleed a loop before you can use it.
Yea, you can watercool and move a basic WC rig. It's a bit heavier but just as durable as any other PC. You could never ship it with liquid in it though, but move by car on a back/front seat? Sure.
Look through some of these internal WC rigs. You can see what's involved.http://www.xtremesystems.org/forums/showthread.php?t=223835