Your own resort will cost you a minimum of a couple million US by the time you get everything up and running unless you're looking at a mom and pop/boutique operation off the beaten track on the mainland.
I came here with the same idea in mind and quickly had my eyes opened to the logistical and financial roadblocks.
Luckily enough I stuck it out, came up with an original idea and made it work. That's your best bet, not trying to compete with existing business. Hotels, where occupancy rates are going down due to the large number of new rooms being added, are one of the worst ventures to get into.
The quote "if you want to make a million in Belize, bring 2" is still alive and true. Bring new ideas and create jobs for Belizeans and you may just do fine.