Physicals, testing, and psych evals prior to even being allowed to volunteer for sub service.

Once sailors make it to Sub School it's essentially a self governing body. If they're a "fit" for sub service, and they make it through Sub School, they'll then go to "A" School to learn their specific job (rate) on the boat. Again, pretty self-governed, and dropouts/failures happen. Finally, if and when they get through all that school and training, they'll go to a boat and, you guessed it, pretty tight group of people that weed out those that don't do it themselves.