Blackadore Caye is a 45-minute boat ride from Philip S. W. Goldson International Airport in Belize City and a 15-minute boat ride from San Pedro, the nearest big town. The island has been used for hundreds of years, according to Juan Rovalo, a biologist who leads a team of scientists studying the caye.

It was a popular spot for fishermen, who would stop on their way to markets in Mexico and cut mangrove, using the wood for fires to smoke their catch and the conch that they took from the reef, littering the island with thousands of empty shells. More recently, he said, the island’s once plentiful palm trees have been uprooted and used to landscape the grounds of hotels in San Pedro.

The villas for guests on Blackadore Caye will be built atop a massive platform that stretches in an arc over the water, with artificial reefs and fish shelters underneath. A nursery on the island will grow indigenous marine grass to support a manatee conservation area, and mangrove trees will be replanted, replacing invasive species. A team of designers, scientists, engineers and landscape architects, some of whom have spent more than 18 months studying Blackadore Caye, will monitor the resort’s impact on its surroundings.

“The main focus is to do something that will change the world,” Mr. DiCaprio said. “I couldn’t have gone to Belize and built on an island and done something like this, if it weren’t for the idea that it could be groundbreaking in the environmental movement.”

An avid scuba diver, Mr. DiCaprio first visited Belize in 2005 to swim its barrier reef. “As soon as I got there, I fell in love,” Mr. DiCaprio said. “Belize is truly unique. It has the second largest coral reef system in the world, and it has some of the most biodiverse marine life, like the manatee population and almost every species of fish you can imagine. Then there are the Mayan temples and the culture.”