MANILA — As the central Philippine island resort of Boracay prepares to bar tourists for six months beginning Thursday, residents fear the militarization of the popular tourist hot spot as President Rodrigo Duterte vows to clean up what he calls an environmental “cesspool.”

Mr. Duterte ordered the closing of Boracay early this month, saying that the water around the tiny island “smells” and that it poses a danger to people’s health.

With tourism the main source of income for its estimated 40,000 inhabitants, Mr. Duterte’s assertion threw the island, famed for its fine, white sand beaches, into disarray. Last year the island drew a record two million tourists who spent about 56 billion pesos (more than $1 billion).

But the scramble for tourist dollars has also fueled rapid development, which has taxed the island’s fresh water supply, power grid and systems for disposing of sewage and other waste.