Correction we had 40 not 4 bars
What do you mean by 'back'? Why not surface before? Draining the tanks to the last bar doesnt sound logic of its a calm open water spot?
Yes absolutely right, that makes more sense, it was a

show of an insta-buddy situation and alignment of bad decisions on both of us; I don’t wanna derail the thread by it but I need to provide context so here goes: my (just assigned for this dive) buddy was not OOG because of any leaks, he didn’t change tanks or bother to check his spgs before going in (I learned that after the fact)
Just as we got to the 30m drop I saw one of his SPGs from the side so I asked him about it, surprise!
We then argued shortly that he wanted to go back alone so I can complete the dive tagging to others in the big group and that I wouldn’t trust him to do that — and I’d join him back, it’s still a long way back in the shallows. He even tried tried asking the group leader (who had dil-out tanks for CC) to switch with him so that he can continue diving who swore at him through the loop some gibberish (never had I seen body language so eloquently say “are you F’in mad!?”)
Having 2 fresh tanks I had 150bars and some peace of mind.. (and this is where I F’ed up) I passed him one of my tanks so we can both be comfortable..
Now it’s not a short swim back to shore, and it’s “calm” but there’s a constant rip tide that’s not ideal for swimming to the exit — so we decided to take our tanks as far as they can carry us
So we dove back (mistake 2 on my side) fullfilling this prophecy to the letter
The whole group swims around lost until they are all low on gas and finally the first diver goes OOA and they all rapidly perish shortly thereafter.
About 100-200m from shore exit we had 15 bars on both my tanks and 10+0 on my buddies; that’s when we surfaced and swam out.
I did get a deserved strong word from the fill guy at the shop about draining the tanks that low.
This was my 3rd SM OCdive after training, and I did acquire some complacency after finishing dives with 100+ bars (still not like my buddy) and had a “relaxed” attitude towards the OOG situation.. lesson(s) learned