“There are older and fouler things than Orcs, in the deep places of the world…”

–Gandalf

It’s dark so requires torches (more than one as a backup) and very easy to get disoriented. You can easily get lost and run out of air. Risk of being blinded by silt even with a torch, leading to more risk of disoriented and getting lost. If anything goes wrong such as equipment malfunction then you don’t have the option of going to the surface as you do in open water (albeit with the risk of a bend). It’s often cramped with places to get stuck, snag equipment, or get tangled in your guideline. There are sharp rocks you can hit your head on.

Cave diving is a completely different skill set than open water diving. While they both are underwater with diving tanks, cave diving takes specialized skills.

Basically yes. Once you go inside a cave like this, it gets dark real fast. You can’t tell where “up” is and you can’t find your way back. So these people often drown or suffocate.

In cave dive training, you learn how not to do that.

Correct, with no visibility it’s very hard to orient yourself

I reckon it was all the water that killed them