All this aside, you can go to a hardware store and buy a couple of self threading sheet metal screws. Little bitty ones and very short. Take a drill and put like a number 60 hole or perhaps like a number 50 in the valve body, through the cap and through the inner flange. Then simply thread the little screw in. And now it cannot come loose again.
The diver did the right thing. But it is evident he was grossly overweighted. Otherwise he would not have sunk so quickly to 30 feet as per the OP while finning up. The ABC agencies teach snorkels, not SMBs, they teach lead, not buoyancy control via the lungs, they do all of their skills over weighted kneeling on the bottom of the pool, they teach the BC as a crutch for being a non swimmer and uncomfortable in the water as if it were some sort of PFD or safety device, which it is not, never was and should not be relied upon as such.
I have gotten lazy and tend to just grab some lead out of the bucket but I also have jettisonable weight more than enough to establish positive buoyancy at any point in the dive. To do otherwise is asking for a Darwin Award, JMO.
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