I got certified for SCUBA diving as a young man maybe 30 years ago, and never continued it for misc. reasons, but ...

They are reporting that many of the boys simply cannot swim. That alone is a concern. Fins might help a tiny bit, but not sure they are practical for the tight spaces of cave diving.

I would imagine that panic is the biggest risk factor in trying to swim out. Boys who are unfamiliar with the very cramped environment that are wearing a mask and using SCUBA are very likely to panic. In their panic they might pull the mouthpiece away while still underwater, and in those tight spots there can't be anyone there to help them.