MIAMI (AFP) - A family of treasure hunters has found booty worth an estimated US$1 million (S$1.3 million) from an 18th century Spanish shipwreck off Florida, the company that hired them said Tuesday.

The find includes 51 gold coins, 12m of ornate gold chain and a single coin called a Royal that was made for then King Felipe V.

The latter is the most important piece of all because it is so rare, Brent Brisben of 1715 Fleet - Queens Jewels told the Florida Today news website.

That firm does treasure hunting work and also farms it out, and one of its subcontractors is a family called the Schmitt's.

The family discovered the treasure a month ago in shallow waters off the town of Fort Pierce, working the Atlantic from a ship called the Aarrr Booty.

But their feat was kept secret until this week so as to coincide with the 300th anniversary of the sinking of a fleet of Spanish ships in a hurricane off Florida, in late July of 1715. The vessels were sailing from Cuba to Spain.

Keeping quiet was "particularly hard for the family that found it. They've been beside themselves," Brisben said.

The treasure is in very good condition, he added.

"One of the most amazing recoveries in 1715 Fleet History. Congratulations to the entire Schmitt family and the crew of the Aarrr Booty," the company said in a Facebook post.

After the fleet went down in 1715, Spain recovered much of the sunken treasure. But hundreds of thousands of coins remain on the sea bed. 1715 Fleet - Queens Jewels estimates that, at a price of US$250 per coin they are worth US$550 million.

The company has made other finds stemming from the 1715 storm.

Under Florida law, the state keeps 20 per cent of the treasure and the rest is shared evenly by the family and the company.