I would guess that TM was just using the most convenient method for himself and TC was being nice and helping him out. Magnum only had to carry his surf ski out to the lawn and get picked up, versus transporting the surf ski to the dock or paddling out to meet the King Kam out in deeper water.
In thinking about the cost of the boat versus the helicopter, if we’re only talking about fuel, depending on how far each has to go, the boat would likely cost more. The Makai Research Pier, where TC’s chopper is shown to be based is very close to the actual estate in reality, just down the highway. If the boat is slipped in Ala Wai Yacht Harbor as has been shown, that’s quite a distance around the southwest coast up to Waimanalo. I’d estimate it at over 15 miles. Kaneohe Yacht Harbor north of Waimanalo is no closer.
It’s also quite a large boat, identified as a Bertram 70, if I recall. It’s a displacement hull with twin diesels perhaps burning as little as 20 gallons per hour at a slow cruise of 10-12 knots, which would take an hour and a half to get there, to over 60 gph at twice that speed. The size is what really makes it so much more costly. It would be far less expensive for a single person to charter TC’s chopper for a two hour flight than to run that boat for two hours for just one person.
A buddy with an inflatable or a jet ski could have taken Magnum out, towing the surf ski behind, for just a few dollars in fuel cost! I know that for certain. I once towed a friend of mine back from near one of the oil platforms here in Long Beach using my inflatable. He was in his wooden kayak that he built himself! His paddle (which he didn’t make) cracked, and then broke completely so he called me from his cell phone. It took me about a half hour to get out to him but it probably would have taken him a couple of hours to get back with the damaged paddle. He picked up the bar tab that afternoon for me and a friend we met at the ABYC. I think I got the better end of the deal. It was a nice day on the water and at the club.
