I dont know if this is the proper subforum but here's the story. Was doing a dive last week with an experienced dive buddy. Surface chop was a bit rough. The boat did not use a tag line for divers waiting their turn in the water. My buddy and I both ascended and I proceeded to board the boat. After sitting down, I looked and saw a different diver boarding. I figured the guy just skipped ahead of my buddy (as happened to my on a previous dive). I didnt see my buddy but figured it was the choppy water and my angle of vision obscuring him. He did board a minute or two later and turns out the rear shoulder dump on his Mares BCD came completely unscrewed allowing the air out and water to fill his BCD. He didn't realize what had happened and kept sinking despite repeatedly inflating his BCD and finning. After dropping 30ft, he dropped his weights and surfaced. Once on the boat, the failure became apparent. The BCD is only a few years old and had been serviced about a year ago. How often does this occur and how does one prevent it from happening?