But they quickly realized they had stumbled across something a whole lot more valuable in the ancient Mediterranean harbor of Caesarea.
Their chance discovery a few weeks ago led to a trove of nearly 2,000 gold coins that had languished at the bottom of the sea for about 1,000 years, the Israel Antiquities Authority said Tuesday.
It's the biggest hoard of gold coins ever discovered in Israel -- and it could lead to further archaeological finds.
"There is probably a shipwreck there of an official treasury boat which was on its way to the central government in Egypt with taxes that had been collected," said Kobi Sharvit, director of the Marine Archaeology Unit of the antiquities authority.
He offered other theories about the origin of the treasure.
Perhaps the coins were meant to pay the salaries of a military garrison in Caesarea, Sharvit speculated, or came from a merchant ship that sank while traveling from port to port along the Mediterranean coast.
Marine archaeologists are planning to carry out salvage work at the site to find out more.
Coins from Shiite caliphate
The coins themselves come in several different denominations and are very well preserved, the antiquities authority said. The oldest of them is a quarter dinar minted in Palermo, Sicily, in the second half of the ninth century.
Most of the pieces, though, are from the Fatimid Caliphate, the Shiite Muslim empire that ruled large parts of North Africa and the Middle East around the turn of the first millennium.
Sharvit said he believed the coins, of various dimensions and weights, had been uncovered by winter storms.
He thanked the people who found the treasure -- members of a local diving club -- for quickly reporting their discovery rather than trying to keep the coins for themselves.
"These divers are model citizens," he said. "They discovered the gold and have a heart of gold that loves the country and its history."