This is a very interesting case. Scuba divers are generally not the type of people whose disappearances go unnoticed. They tend to be middle class; particularly if they own their own equipment.
Hawaii is a popular place for Scuba. My guess is that this guy is not from Hawaii as the water there is not very cold and the type of suit he was wearing would 't be necessary. Many serious divers prefer to transport their own equipment (except perhaps the tanks and weight belt which are quite heavy) to where they dive so it seems most probable that he was from the mainland and brought at least some of his his equipment with him. Diving suits are seldom purchased used so the guy probably bought it in Tacoma so it is very likely he is from the greater Seattle area.
Most Scuba Tourists in Hawaii hook up with an outfitter who can rent them any equipment they need and then take them on a boat with other divers to a desirable diving spot. The ones I have been on carried 10 to 15 divers. These operators are pretty professional and have liability issues. It is always possible that one diver will separate from the others and get in trouble and die, but the operator would know it and initiate a search. It seems close to impossible that an operator would depart without knowing that one diver was missing but it happen once in Australia. I seriously doubt that happened here.
Sometimes divers go on " do it yourself dives" from the beach. This is cheaper and many off shore reefs are well worth seeing. The big danger in such dives is that you can be carried by the current further from the beach than you expected and run out of air before you get back. A particular problem in Hawaii is that impossible to climb cliffs are often right next to easy beaches and swimmers or divers find themselves trapped by the current. Diving alone, even for experts, is considered dangerous. Beach dives are particularly dangerous and are usually done in larger groups. Sometimes very confident divers take foolish risks.
My guess is that this guy was a diving tourist who secured full tanks and took his rental car to a beach where he took a solo dive and didn't make it.
The rental car would eventually be recovered and all the expenses charged to his credit card. Any equipment he rented would be billed the same way. The hotel would hold his cloths and luggage a few days and then get rid of it but no one in Hawaii would be likely to report him missing. Back in Seattle or wherever he was from, friends, his employer and landlord would know he was missing but would they report or would the police even take a report from a non- relative?
Before the national data bases were set up and reporting methods standardized it was very possible for a "normal" person with normal social, work and family contacts to disappear and not turn up on any data bases.