I'm not sure you're getting a full picture of the Industry from the posts here, and I'm not an expert so could be well off but I would say it's a pretty competitive market. All of the major marine compaies are represented here via Agents that I pressume in true Belize style, have exclusive in country contracts. ie Honda, Yamaha, Suzuki, Mercury etc. These retailers are pretty much all large Companies (for here) and in Belize City and each has its different manufacturer. In Belize there is also Duke Marine who build boats, trailers and mechanic shop(?) and is also a general Marine Store. Cucumber Beach Marina is in Belize (the largest and perhaps only one) who will also offer similar and more specialist services.
Here on the Caye we have our own West Marine in Captain Sharks who have a large range of products and also have a Boat Yard where they build boats, do general repairs, paint shop etc. A lot of the Hardware Stores also carry an ever growing amount of Marine Products too. There are a handful of other "Backyard" Boatyards who will do fibreglass repair, painting and mechanic work and keep busy. There are a handful of certified mechanics on the Caye that are company trianed and affiliated to the Belize City Dealers, but independant - the two I have had dealings with over the years are Osmin and Caesar, and both have been great. I personally know both are certified Yamaha mechanics (my engine is a Yamaha) and have travelled as far as Japan for detailed training. There are even more non certified great mechanics, especially with the two strokes, as just about every fisherman on the Caye has been working on their own engines for decades before the current level of support was there.
So as you can see it's a pretty vibrant industry as there are a lot of boats in Belize per capita - you'd expect nothing less with such beautifull Water. Room for another? Sure. It at least gives you a few more contacts or names to look into and if you have a specialist skill that could enhance one or perhaps offer a new service company.