My worst experience was years ago, diving with an operator that contracted with the Cruise Lines to take Cruise Ship Divers. Because I was diving solo, I was paired up to be a dive buddy to a diver off the Cruise ship, a woman in her mid 40s who was certified, but had not been diving in 5 years and had never done drift diving. We were going to Santa Rosa Wall (a great dive, and not terribly hard, but I don't think a Dive Op ought to take a diver there if they have no idea about his or her skills). This woman was a disaster from the start, overweighted, BC didn't fit and kept riding up, couldn't adjust her buoyancy, and, despite a decent dive briefing on the boat ("stay close to the wall, there may be down currents, pay attention to your depth"), she was totally oblivious. I was trying to keep my eye on her, and stay close, but after looking at something on the wall, I turned to find her, and she was 25 or 30 feet away from the wall, 20 feet below me, and I was already at about 95 feet (max depth on that dive was 100). I went after her - she was fiddling with her BC, not paying any attention to the group or where she was, and continuing to descend - and caught up with her at 135 feet. I dragged her back to the wall and grabbed her console to show it to her -- she had no idea where she was. I got her back up to the group and spent the rest of the dive babysitting her. Even at the end of the dive, when I bitched at her about not paying attention, she had no clue that she had been totally oblivious. I swore off diving with any Dive Ops that took divers off the cruise ships. I know, some of them, maybe many of them, are competent, careful divers. But I didn't ever want to be paired up again with a disaster waiting to happen.