By Peter Polack

GEORGE TOWN Cayman Islands (Reuters) - A small, homemade aluminum boat carrying 16 Cuban migrants sought refuge from rough seas in Grand Cayman on Thursday but local officials prevented a landing, citing a migration agreement with Cuba.

Local law enforcement would not allow the migrants to receive drinking water, food or supplies from well-wishers onshore. The 20-foot boat, a patchwork of fiberglass and metal with large inner tubes attached to makeshift outriggers, was allowed to anchor offshore while their fate is determined.

Under the 1999 migration accord Cuban boats are allowed to pass through Cayman waters as long as they do not seek any assistance. If the migrants come ashore, they are taken into custody and usually repatriated to Cuba.

Boats smuggling Cubans seeking to flee the communist-run island are frequently seen off the Cayman Islands, located in the Caribbean less than 100 miles (160 km) south of Cuba.

They are usually headed for Honduras, about 400 miles away, from where migrants make the long journey overland to reach the U.S. border with Mexico. Under the U.S. so-called "wet foot, dry foot policy," Cuban migrants who make it onto United States soil are allowed to remain while those intercepted at sea are turned back.

"We left (Cuba) because there are no jobs or the basic items for living," said the boat captain, who identified himself as Angelo. "We built the boat in secret near the sea and we left at night to avoid detection," he added.

The captain said the passengers, 11 men and five women aged 18 to 40, were from Manzanillo in eastern Cuba.

He said the boat had been at sea for five days since leaving eastern Cuba, surviving rough seas whipped up by the passage of hurricane Cristobal to the east.

The boat had no shade from the blazing summer heat and the group appeared to have run out of water.

Cayman officials confirmed that immigration officers were at the scene. "It appears that two of the migrants were here before and were repatriated in early January," said spokesperson Bina Mani.

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Estimates are that 200 to 300 Cuban migrants have passed through the Cayman Islands so far this year and stopped for various reasons including bad weather, engine problems or lack of water.

Angelo estimated only one in 10 boats make it to Honduras.

Local islanders calculate that at least double that number pass near Cayman Islands without stopping, and others are intercepted by the Cuban government vessels and the United States Coast Guard.

The U.S. Border Patrol said in late July that more than 13,500 Cubans without the proper papers had tried to cross the southwestern U.S. border since Oct. 1, 2013, more than during all of the previous 12 months. Four years ago, the 12-month total was about 5,500.

(Reporting by Peter Polack; Writing by David Adams)