Ambergris Caye is not a very cheap place to live, not by a longshot. It costs a lot less to live on the mainland, but that won't work for the business you want to get into.

To paraphrase what has been said here over and over and over again:

"how do you make $200,000 in US assets in Belize? Start with $400,000"

That said, perhaps if you are some kind of amazing business person at home, you'll also be an amazing business person in Belize. It could happen.

I could be wrong, but aren't there enough dive outfits in A/C? I know there are here on the peninsula. Just sayin'.

Yes, there are more than enough dive operations here and some are starting to undercut others.
You might do better looking at an area other than Ambergris Caye - maybe one of the atolls. However, that would be a problem for schooling. However, a lot of kids here do home schooling aided by the internet. Where there's a will there's a way - if you do your research and use good sense.

Sell the boat, put the money in the bank, take your time to look around and visit all areas of Belize, live off the interest in the meantime (6% for US$ accounts).

Ambergris Caye can be very affordable, it can also be very expensive, it's all relative.

Do a load of research on the web (and here is a good start) then do a trip for several months minimum, before committing and stay in a couple of different spots to see how you like things in each even if you think you know where you want to be. We did the same when we first arrived and from our research thought Placencia was our likely destination. Once there we realised San Pedro was a better fit for us and we only realised this as we had stayed in San Pedro on the same trip.

An imported boat will cost loads to get here and incur Duty at around 50% of it's value. A big start up cost.

Never assume anything. If you had friends move to Belize three years ago and you lost touch it is more likely they left than stayed. I would guess a third of those that move make it into year 3-5. Perhaps less.