The risk of diving.
One of the most dangerous jobs onboard submarines is open water SCUBA diving. You might ask yourself why that is since the submarine does so many evolutions that are dangerous. One of the main reasons is that when you are at depth and working there is no direct backup if things go wrong. You are absolutely reliant on your equipment, how well trained you are and how proficient you are. If SCUBA divers become complacent and do not maintain their level of knowledge and proficiency in all aspects of conducting a safe dive there can be many unintended and negative outcomes.
Submarine SCUBA divers face many unique challenges and I will talk about a few:
Diving as a collateral duty. There are many day to day activities that conflict with diving. Some of these activities are ship control trainers, attack centers, daily maintenance, etc. With all of these primary duties requiring action, collateral duty SCUBA divers do not have the opportunity to dive as much as someone whose primary job is diving, leading to a lower overall proficiency level. This lower proficiency level does not automatically mean unsafe practices or lack the ability to perform the task, just that the divers might not have the experience gained by "on the job training" and working at different diving commands (salvage commands compared to underwater ships husbandry commands).
Watchstanding and maintenance. These two roles make up a large portion of what the ship is involved in while it sits next to the pier. When one watchstander is pulled away from duty to dive it affects about five other people who are on the watchbill or need to stand in for the diver.
Dive site setup and gear stowage. Dive gear is stored in many different places some of which are difficult and time consuming to get to. This leads to longer setup and breakdown times for dive sites. This also makes the proficiency dives harder to fit into a normal workday due to the time commitment and the affect that hanging divers tags has on the ship.
Lack of planning, equipment failure, operator error, and other factors all contribute to diving being an extremely dangerous evolution. As recent SCUBA fatalities have shown SCUBA diving is very dangerous and the risk of death is ever present. Even routine evolutions such as hull searches, retrieving dropped equipment, or just performing proficiency dives. Commands need to be cognizant of the experience and proficiency level of their divers and dive supervisors. It is not a bad practice to get local divers, who do this for a living, involved to help out if the divers are not proficient enough.