An odd decision, I would have thought he was a good diver, so presumeably also a decent free diver, knew the boat well, could alert rescuers to the trapped people, and would not use up oxygen when gone from the bubble.
On top of that, even if this guide was a Divemaster which is far from guaranteed given the other safety failings from this company, being an advanced Divemaster does not equate to one’s ability to cave dive. Survivors state the lack of lighting in the boat and so I’m assuming he would have had to navigate the boat in complete dark under water. So while not exactly cave diving, it’s equatable. An activity that’s insanely dangerous to anyone, even Divemasters that don’t have cavern or cave experience.
There’s a reason there’s a grim reaper sign at the entrance of caves in popular dive locations.
Still, deciding that the guide should not try does not even make sense if there was only a 5% chance he'd make it, from the two tourists point of view.
Edit: ah, name checks out.
Noises are also very different above and below water - even when you’re in an air pocket. They knew they were still at the surface and trapped in a capsized vessel.