They are elegant and effective. During any open nozzle* launch, the water is expelled in a tiny fraction of a second. Acceleration is almost immediate, and the rocket's flight is almost entirely ballistic. This all starts with both the rocket and it's 'fuel' standing still. With a tube launcher, the initial thrust takes place with very little loss of water, and lasts the length of the launch tube. At that point, not only the rocket but also it's considerably more massive fuel are already traveling at speed. Any loss of pressure up to this point is partially offset by air entering the rocket from the launch tube.