There is a difference between lagoon properties and West coast properties which are acually beach front (and some nice ones too!)
Mr. DiCaprio is not a hotel developer, and the fact that he has maybe signed a contract for a hotel chain to open an eco-resort on land owned by him (on another island), does not automatically increase the property values of any land on Ambergris Caye at all.
Don't get caught up in the name-dropping tactics. A swamp is a swamp is a swamp, no matter what kind of movie star has set his foot on an island in the waters nearby.
Having said that, there are several great deals on lagoon (and west coast) properties. Just don't think that Leonardo is going to make your land value sky-rocket. That would be like claiming that the south end of A.C. became fancy because they filmed a couple of scenes with Harrison Ford for the "Mosquito Coast" movie there back in '85 or '86.