That would have absolutely terrified me since being lost at sea is one of my worst fears and I used to have recurring dreams about this. I'm sure sitting in a pitch black, cold place for several days would have made me go insane. But I bet the diver was even more terrified when he was down there and someone grabbed his arm, which I'm sure felt like a scene out of Event Horizon.
But I don't understand why the guy was just sitting in there, pretty much accepting death. If your life was on the line, you don't go down without a fight, just like if someone busts in your house at night, you don't hide in the closet or under the bed. So if any potential burglars are reading this, don't come to my house unless you're prepared!