I too was in subs, the problem here is that you will not always have enough people to man a three section rotation. People cycle on and off the boat very frequently I spent 4 years on the same boat with a crew of on average 160 people. In my time probably 300+ people rotated through, routinely many divisions would have only 2 qualified individuals for a watch station putting them on 12 hour shifts with having to get up in your off time to give a food and bathroom break. The issue is the military goes through periods where a certain rating is undermanned so they encourage the recruiters to take as many people for that rate as possible. Since recruiters have quotas they must fill they take people who are quite frankly useless. Those people end up not able to advance due to several factors and the Navy can only have so many people in a rate at the same pay grade so they either don't let them re-enlist or if they want to stay make them change rates to an under-manned rate. Now not all those who re-enlist will advance due to the same issue so often they just cut there looses and move or they reach higher tenure and can't re-enlist leaving an under-manned rate and the cycle repeats itself.

Most watch stations take months of training to not only get qualified but also for the command to have enough confidence in you to stand the watch. I myself stood a regular 12 hour rotation during times we where on the surface because we had only 2 qualified operators and the rate that actually owned the equipment was undermanned so they could not provide bodies. There are very rarely extra people to do grunt work you do that in your off watch time when you are not studying, qualifying, or doing maintenance. Very frequently commands will care about filling the watch bill over a person not just being qualified but also competent enough to stand a watch, thus accidents happen due to negligence and ineptitude.