The availability of water on the boat depends on what the boat is doing....during quiet ops, the water making equipment is secured. There were plenty of times when we were underway when the showers were secured to conserve water.
BTW...submarine showers are non-water running showers....you only have the water running enough to rinse. And yes, you can get in trouble for taking a long shower. I, on the other hand, got to take all the long showers I wanted-—’cause I made the water.