Prepare yourselves because I am about to sound like a moron, but I have to ask: if it’s a training exercise, why wasn’t an instructor monitoring it at depth?
Please be kind.
There were disciplinary actions taken. From the daily Mail article”
“..The deaths resulted in at least five members of personnel being disciplined. Four were charged with dereliction of duty, and the commanding officer who was unreachable was fired for being ‘unapproachable and disengaged.’ “
It wasn't that type of "training" where an instructor is on hand showing you what to do.
These two men were seasoned divers and they were practicing the retrieval of a downed helicopter for an upcoming mission.
You might ask how could they drown after only being under water for 31 minutes with a scuba tank.
Here's a rough explanation: Let's say for example their individual air tanks contained enough air for an hour dive. On a typical dive, the deeper you go, the surrounding water pressure continues to compress the oxygen in the tanks. So while diving at let's say 25 feet will give you an hour's worth of oxygen, at 150 feet of depth, the oxygen has been compressed to the point that it will yield only 30 - 40 minutes of breathing time. (Again, this is only for example purposes only).
Diving is a science which deals with the "partial pressure of gasses". I'm sure you heard of "nitrogen narcosis" or the "bends", both inherent dangers when diving below 100 feet.
The air you breath is actually 79% nitrogen which does not leave the body during normal respiration as the O2 (oxygen) does.
So when you have been diving at depth (150 ft.) your body has just consumed an hour's worth of oxygen and nitrogen in just 30 minutes so now you have to go thru decompression at various depths to rid your body of the nitrogen that has built up before you can resurface at the top.
This is just a ridimentary explanation for you, hopefully someone here with extensive diving experience might stop by and give a better explanation.