> My dive buddy took a minute to realize what I meant and then another minute to fumble for his spare regulator

(emphasis mine)

Yikes! Why didn't he let him share his main regulator while he was looking for the spare?? What if he couldn't find the spare?... (they often become entangled in the tubes in the back and are hard to get)

This is really bizarre.

So many questions (and so many places this could have been fixed). a) As soon your dive buddy saw your air gauge was that low he should have gone into the share air procedure. Parent's idea of letting him have your main reg while fumbling for the spare is also a good idea (although I don't believe most training includes that step). b) The divemaster should have been proactive about asking you how much air you had. It is really easy to suck down a lot of air quickly (especially your first few dives) and asking the group how much air they are using early on can indicate if someone is being aggressive with their air. c) i can't believe you guys were spread 60-70 (in total) away from each other. That's pretty far. d) If you were that low under where you ran out after 10 feet, the DM should have taken you up. Remember you also need to inflate your BCD on the surface, you don't know if the boat is going to be there, lots of things can still go wrong.

I've been in a really similar situation. 100 feet down, my dive buddy was someone who hadn't dove in 5 years. It was a drift dive and he was trying to take a lot of pictures. 20 minutes into the dive (the DM queried, he didn't volunteer) he revealed he had roughly 500 PSI left (some believe that's the level you should be at when you get out of the water). Our DM sent us up together, we shared air ... but it was still touch and go.

"Forgot that air supply runs out faster the deeper you are."

I ran into this one on my certification dive; we were diving on the Madera in Lake Superior in December, I was having trouble keeping my bouyancy which I chalked up to inexperience but later discovered was a leaky BCD. When we went to move to the stern which is in 60 feet of water we all checked our air and I had a quarter of a tank left! Everyone else had at least three quarters of a tank. None the less, the rest of the dive was called off and we had to swim back to shore. I asked why we didn't at least try the deeper dive and he reminded me that you consume much more air in deeper water, pretty much double every 30 feet, that we wouldn't have even made it to the stern before I ran out.

The divemaster should have rejigged the buddy pairs so each newbie was with an experienced diver.

A lot of new divemasters are overconfident too, it's possible to go straight from scratch to DM in a summer, but it's like that SCRUM master certificate doesn't make you an expert methodologist by itself, you need experience too.

Actually, my dive buddy was pretty experienced, and I think that was part of the problem...he was just complacent. You see the same thing with a lot of pilots who have several hundred hours under their belt. They have enough experience to be overconfident because they haven't had the shit scared out of them, and not enough to know what they don't know.

An amusing story from my last dive trip, where the DM had done his training a closed fist meant 0 whereas we'd all been trained that it meant 50. So for the first couple of dives before we'd asked why we were coming up early, he'd thought everyone had 50bar (a quarter-tank, for the non-divers) less than they had...

So, you hadn't dived in a year, since your cert. You planned to be in over the cert'd depth, on your first post cert dive. In a group with one dive master, over a wall, which is tricky. And then there was peer pressure to not suck down air too fast. That's not really a recipe for success.