You've all given me some things to thinks about for next time.
To fill in some info.,
-The manager of the resort accompanied us on the trip and helped divers gear up and get back into the boat. The kids were under his supervision in the boat and were calm and comfortable when we returned. Conditions were good. Calm seas, no current, good visibility.
-When son had to return to the boat, hubby motioned to me to stay put and went up with son. I hovered below him at about 10 ft. while he watched son swim to the boat. We did a few visual check-ins and I used the time to clear my ears. The instructor was within sight but was concentrating on daughter.
-As soon as we got to the reef we found New Guy and continued from there as a group of four.
-Instructor was interviewing for a position as the resort's full-time DM. It's very possible he did not feel in full control of the situation and didn't feel he had the latitude to limit the group, especially as New Guy had been okayed by the manager.
-Instructor was out with son and hubby the day before so he'd had a chance to see how they performed in the water. He had complimented them on how they worked as a team and on son's skills in general.
-New guy was well aware he was going out with a DS group. He'd seen us in the pool earlier in the day and I commented to him about this being my first real OW dive as we were getting on the boat.
-I think one of the reasons we agreed to have son buddy with new guy was that we didn't want him to have to stick to a DS group. I was comfortable with the idea of son (certified, 20 boat dives), diving within sight if buddied with another competent diver. As some of you pointed out, he should have been buddied with husband, I should have been the responsibility of the instructor. Our mistake. I think I was hoping Instructor would be able to concentrate more on daughter if he knew hubby was with us and helping me to keep track of the simpler stuff.
-I'm not convinced New Guy was the diving stud he presented himself as. He either didn't watch his air or ignored the instructor's instruction to come up at 500 psi, as he was well below that when hubby indicated it was time for him to go up, and when we returned to the boat he tried to take off his BCD before removing his weight belt.
Some of the things I can see to be wary of in the future:
Last minute changes to a plan made in advance. The original plan made sense. As things got more complicated the chances of a problem increased.
Bending the rules. Son should have been with father. DS dive shouldn't have been mixed with general recreational dive.
Diving with an unknown buddy without spelling things out in advance. While I did talk to the New Guy and got a conversation going between him and son we made a lot of assumptions, both about his skills and his willingness to act as an appropriate buddy to a teenager.
Daughter and I are planning to do the full course this spring. Hopefully we'll have many more opportunities for successful dives!