The 5th guy worked for the company that owned the salvage rights to the Titanic, was a professional diver, and was considered an expert on the Titanic, and he'd been on a lot of dives before to recover artifacts and map the wreck.

He more than anyone should have known this was the crappiest sub he'd ever been on, but he was seemingly obsessed. I can imagine he was invited and didn't pay for a slot.