OK, I haven't done this exact thing, but I do have an anecdote to share. I'm a programmer for a large 'blue' company and have been tired of my job for years. Unfortunately, it pays the bills, I'm good at it, and I'm really not good at much else that pays money. But I digress. The anecdote:
Back in my college days I was casting about for a job. Not just any job would do, however. I wanted something that I enjoyed doing and that interested me. I was a steady customer of a pet store in my area. I love animals and thought that working at the pet store would be a great job that I would enjoy going to every day. They offered me a job and I jumped at it.
After three days of having to show up at 4AM and cleaning mass amounts of filthy cages while the animals were jumping up on me, I realized that the reality was not nearly as appealing as the idealistic version that I had pictured in my head. I quit and found a job at a video store. Now the only animals that I got to work with were irate customers that didn't think the rules applied to them, but on the other side of the coin, I didn't have giant loads of guano waiting for me each day when I walked in the door.
Anyhow, I don't have any experience working in the dive industry, but I have to assume that there are many aspects of DMing or instructing that would just plain suck. If I had to deal with people like me (and worse!) on a daily basis, clean the heads in dive boats, babysit, watch idiots breaking off coral for souvenirs, etc., etc., I think it might suck the fun right out of it for me. One thing that I've found in my adult life is that the easiest way to turn something you enjoy into something miserable is to do it for a living. Especially if you don't know exactly what you're in for.
Then again, what do I know? There's a reason that I write code for a living instead of dealing with real live people

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So I guess my advice would be to talk to absolutely everybody you can who does what you want to do and learn everything you can about it. Who knows, it may turn out to be your dream job! I sure hope so. Best of luck with whatever you do...
David