Your description has them re-boarding the boat alive. Complete newb nobody here but that makes it hard to tell what you're asking. In a strict logical sense only health issues on the surface or non-diving boat/weather trouble could be an issue at that point. I think maybe we are supposed to imagine things that could happen to interrupt this story before their return to the boat?
Hardly.
==== the story of Bob and Karen ====
During the ascent Karen was trying to take Bob's picture with the sun for back lighting. She loved silhouettes and Bob's athletic body made a scrumptious subject. She always loved the way Bob looked in a wetsuit.
Not paying enough attention to the control over her buoyancy she started sinking away. She hardly noticed the extra pressure on her ears because of her single minded focus on getting Bob framed just right in her viewfinder. Bob saw her sinking and knowing that this wasn't the plan turned back to go alert her to their need to ascend.
He turned to face her and in so doing ticked the camera with his fin, which in turn slightly dislodged Karen's mask and it partially filled with water. Karen had had trouble with the mask skill during the open water training but the instructor told her that with time and experience her issues would simply go away, but they hadn't. She dived like a seasoned expert as long as there was no water in her mask, but even a few drops were enough to make her feel stressed and what had happened when the camera bumped her mask had let in more than a few drops.
Bob knew this and his heart jumped to his throat as he saw Karen throw the camera to one side and grab her mask with both hands. Her breathing had suddenly gone from slow and even, just like Jeff, their instructor had taught them, to the panic driven panting of someone who was sure they were about to die. The fight or flight reaction was taking hold and Bob knew that he needed to act fast.
He grabbed Karen by her trim vest and tried to stabilize her so she didn't sink any further or float to the surface in an uncontrolled ascent. Karen was trying to clear her mask but just like in the training all of her efforts were useless. Air was coming out her her regulator instead of out of her nose. Jeff had warned her to work on this but she never did because it always caused such a feeling of unease to let water in her mask. Karen had resolved to simply never let that happen.
Try as she might she couldn't clear her mask and the panic deepened. She started struggling against Bob's firm grasp. She needed to breathe. She needed air, blue skys and seagulls, and she needed them RIGHT NOW. Deep in Karen's subconscious she knew it was the wrong thing to do but she couldn't help herself. She cast off her mask, almost demonstratively and started swimming as hard and as fast as she could to the surface. Bob tried to slow her ascent as much as he could and being alert to the need to keep breathing, he watched to make sure that she didn't hold her breath. Jeff had described what could happen if one made an ascent while holding their breath and fortunately that lesson had made a big impact on Bob.
There was no need, however, because Karen was breathing in and out so rapidly that a continuous stream of bubbles was coming out of the exhaust valve of her regulator. This was only her third dive using it. Unaware of Bob's help, Karen struggled like an animal in a deadly trap until she finally broke the surface. Immediately she spit out her regulator and starting making gasping, choking, coughing calls for help. She had forgotten to inflate her BCD and was now in danger, having reached the surface without serious injury, of re-sinking below the water line and drowning.
Bob recognised the problem and reached down to Karen's midriff and unbuckled her weights. They slipped off into the deep and Karen was now firmly pinned on the surface of the water. She was safe. She was safe.
She was alive.
==== end ====
I didn't make it too gruesome. Karen only lost her camera, her weights and her mask and didn't get injured but in principle this was an accident.
If you wanted to rewrite that make her drown or get a barotrauma then it's not much of a stretch.
And the worst part is everything I wrote is based on my knowledge of *actual* events.
The point being that accidents, more often than not are precipitated by seemingly benign beginnings and quickly worsend by skills that were either not mastered or never practiced.
R..
