Wherever you decide to open your operation, the money question is what you'll have to answer under the line.
I'll let you do the calculating for the following:
Investment for the type of operation you want (that definately includes diving gear, compressor, boat, ect), an operation that can accomodate only a few divers will not survive, unless you don't need to earn money...
You'll need to be able to survive with 0 income for:
-al least one year until your operation has legal permits to operate and from there
-if you are in a less developped area, several years until you get this area a diving reputation
-if you are in developped area, several years until you make your place among your competition
-and very important, don't expect any help, unless you pay for it!
If this does not sound like a problem for you, think about the administrational headaches to get your papers, get familiar with the local laws, ...
Of course, as a dive shop owner I have no interest of telling you how great an idea you have, but I'm just talking out of experience.
Once you have a number with this (and I've only mentioned the most important, marketing cost will be considerable as well, salaries ect. ect.), go offer this full amount to any dive center around, because you will probably reach an amount of money, that any dive shop owner would sell his operation for...That way you immediately have a turn key dive center and you can worry about all the other issues that you'll have to resolve like residency (you also want to work in your operation, I imagine...), your daily beach walk, the beer with friends...;-D
The rumor says, that there really is a dive center in the Playas del Coco/Hermosa area that is looking for a partner/new owner...