My first post-cert dive was in Kona, HI. Crystal clear water and 25' max depth. I paired up with a lady who informed me that she wasn't real comfortable in the water, and it took several attempts for her to get through the classes. We agreed that if she wasn't comfortable she would surface and I would continue with the rest of the group. She wasn't, and did, about 2 min. into the dive.
I guess if I had known then what I know now about the risks of solo ascent, I might have escorted her to the top, but, after all, it was only about 15', still right below the boat, and she indicated that she felt OK to surface alone. I watched her 'til she was on the ladder.
I continued the dive, keeping with the group, but without a designated buddy. I felt I was close enough to find air should something happen, and was also comfortable that I could do a CESA at any time. I had a standard set-up with AL80, one first, two seconds.
Since we discussed our plan before the dive, we also could have talked with another buddy team and arranged for me to join them.
Having said all that, I would probably not do anything different in the future, given similar conditions (clear water, no overhead, <30'). I would even consider doing straight solo dives in similar conditions, although I'd probably want a pony. Ive always felt very comfortable in the water, and have never been prone to panic. I believe I could clearly think my way through a problem and choose the best alternative.
BTW, a couple of days earlier on my cert dives (Shark's Cove, North Shore, Honolulu) the instructor told me that he felt like he was diving with a buddy, not a student. We were swimming through lava arches, so breaking the no overheads rule.