I've owned/run/sold a few small businesses (not diving related) and have many years of management consulting experience, and unless you have experience running a small business and/or have worked in a dive shop (and preferably both) you are looking at a very risky proposition. I could write pages about what to look for and how to value the company (and you should pay attention to Clark Fletcher's comments, plus there is a ton of stuff you can read out there in the world), but if you were my client I would spend our first meeting trying as hard as I could to talk you out of this. If you still insisted on going forward after that, then the fun would begin.
Financially, to grossly oversimplify, you need to figure out how much in total you are putting in (cost of business plus additional near-term investment for new equipment, immediate repairs, debts that are near due, etc.) and how much will be left over each year (AFTER paying yourself a reasonable salary, assuming you value your time). (This will require doing a lot of serious forensic accounting.) Divide the leftovers by the total you put in, to compute an annual return on your investment. I earned 14% return last year in the financial markets without doing a lick of work and taking only a moderate risk. You will be working really hard and taking a big risk. Hopefully your number is way more than 14%.
And the best case scenario for buying a small business is to find an aging owner without children who want to go into the business and who just wants to get out, and the business has a great reputation/brand image. And in general, unless you can see something about the business that is undervalued and that is not obvious to other potential buyers, or you have some kind of an "in" with the seller, there will be no bargains.
On the bright side, while I know the owners of many small businesses that have failed, dive shops in the Caribbean all seem to stay in business a long time. But I wonder how much the owners make per hour. Hope it's more than you'd make flipping burgers.
Oh, and not to be harsh, but the fact that you are asking these questions on SB suggests that you might not be ready to take the plunge. If you are serious, hire a professional advisor who has experience valuing small businesses.