There are big things to look at with a question like this. Being involved in the hobby and getting paid is one thing to consider. I'm refering to breeders, wholesale, retail etc.... I've worked that end of it before. Some people really have a nack for it and do very well. If that is your focus, then get into it at any level you can, and become familiar with it. With time, work your way up to the top.
As a professional aquarist/diver, you don't do it for money. Most keepers have at least one part time job on the side, if not more, just to make ends meet. The hours are long, the nights are short (unless you are working nights like I am today and tomorrow - then they're long, too) the pay is minimal and the glory is placed on someone else.
Pay is based on two things. How much the company is willing to dish out, and how much of a demand there is for you. I don't mean 'you' as in your position, I mean 'you' as a person. Take two people applying for a job. Both have met the normal requirements and all of the related things. The position will be focused on (as an example) cold water marine inverts. Now, if you have 20 years of experience, but it's African feshwater time under you belt, you'll probably be looked over for the person with only 5 years of experience, but two of them were coldwater marine. Best thing to do is get BROAD with the range of animals you are familiar with.
Lately, I've been pretty narrow in my scope of work. I've been lucky - the places around me seem to need just what I do. The trend won't last, though. And soon I'll need to refamiliarize myself with more animals all over again.