He was both.
From Wiki.
Navy SEALs weren’t as well known to the general public back then. They hadn’t been elevated to mythological super soldiers of TV and film yet.
And I don’t believe the show ever used the word SEAL in any episode, at least none that I remember. I read that Magnum wore SEAL shorts, but if you weren’t familiar with them, they looked just like shorts. It was an insider clue, if you recognized it (I didn’t).
It would actually have helped the show if they had said it. I used to roll my eyes in some episodes at how much stamina Mags had. Carrying Higgy on his back up 2000 stairs, treading water for 24 hours, completing a triathlon with no real training. It seemed like he was too unrealistic. If I’d have known he managed to get through SEAL training, well, yea, that explains it. It wasn’t the physical stamina so much as the mental stamina the SEALs bring out in you. To know you can do it. To never give up. “Beauty knows no pain.” 
Two posts in a row, but I’ve been thinking about this.
Magnum on the surface was a show about a hunky good looking guy who drove a cool Ferrari and lived in a beautiful beach house, and was a TV private eye (excuse me, investigator). Meaning his cases were interesting and cool and didn’t involve peeping in bedroom windows photographing spouses cheating, which is probably 95% of actual PI cases.*
At a deeper level, Magnum was about the golden boy getting older. Magnum was portrayed as not just any Annapolis grad, but quarterback of Navy. Not some schlub nameless lineman, but The Hero. He then became a SEAL, then had a great military career. Survived two tours of duty in Viet Nam. Became a successful counterintelligence officer.
And threw it all away to become a beach bum. Drove a beat up VW bug. The character is the guy most were envious of and a few hated. But the show is about what happens later. Everyone remembers the school’s star athlete, but no one knows where he is now. You may go 21-30, with 3 TD (2 Pass, 1 Run), 1 INT. and be awarded the game ball by the Secretary of the Navy, but then what? When you’re being brutally interrogated by Russians, or applying for a job, do they care? Do you let it define you? If you’re a shallow person, maybe. Magnum didn’t.
Magnum said he quit the Navy because he realized he was thirty and had never been twenty. When I was twenty watching the show for the first time, I didn’t get that. Now I do.
Somehow I don’t think any of these subtleties will make it into the new show.
*The one time on the show they did have Magnum doing that, the paramour ended up throwing the cheating wife off a cliff. Not your typical divorce case.
Very insightful post, I agree. All of the views of Magnum were true, he was a guy who took advantage of his buddies and never paid his fuel bills until he absolutely HAD to. Those who admired him, and those who were disappointed in him were all correct. He ran at forty percent efficiency when he was peaking, less most of the time. All of Higgins disapproval was earned—but who wouldn’t envy him? He lived an ideal life and didn’t work for it.
The thing is, he had earned it previously when he was serving. Perhaps that is why he went back to active duty at the end of the original series; it was the only way he could completely respect himself and live up to his potential. He wasn’t a fat trust fund baby, he was the guy who put in his time, rose to every challenge, mastered every occasion, and needed a big long shore leave to regain his equilibrium. His investigator career wasn’t taking navy to a bowl game—it was throwing the ball around in the park with your buddies. When he laced up his game cleats (put on the dress whites and re-upped), you knew you were going to see a side of Thomas Magnum you had only glimpsed before; one that even Rick and TC barely remember.
The whole show (in light of your quoted post above) reminds me of the Toby Keith song; I’m not as good as I once was—but I’m as good once as I ever was. Magnum wasn’t a kid who could go for days without sleep anymore, but what he lost in youth he more than compensated for in age and experience. Living off of Robin Masters taught him how to use other peoples resources to solve his problems (like in Judo, he can use the opponent’s force against them). If “Magnum, the Active Duty Years” was a spinoff of the original, it would have a very different vibe (more like Mark Harmon in NCIS- occasionally humorous, but always dead serious; [emphasis on ‘dead].)
The new show is only about drinking with your old war buddies. It looks more like an adaptation of The Three Musketeers, a group of deeply devoted semi-retired soldiers who get in their cups and seek adventure. A group of such competent soldiers and swordsmen they beat all comers with minimal effort and amusing hijinks—all while throwing out wry quips.
I hope I am wrong, but the new show seems a mile wide and an inch deep. They would do better add some depth and not try to shine so widely.