A specialist search team from Somerset, contracted by the 13th Duke of Argyll, have positioned their barge in the waters just off Tobermory's colourful main street.
This will be duke Torquil Campbell's fourth attempt to find the treasure, which is said to have sunk with a Spanish galleon.
There have been an estimated 50-60 attempts by the Campbell family to retrieve the lost fortune over the years, as the duke's ancestors were given exclusive search rights by Royal Charter.
Brian Swinbanks, chairman of Tobermory Harbour Association, said: "It's a fantastic story. One part of me would like them to find something and another part of me would like it to go on forever, like the Loch Ness monster."
Tobermory historian Olive Brown added: "The ship went down in 1588 but the searches weren't immediate. It was into the 1600s before the rumours started that there was this £30m of gold on board.
"The latest techniques from the 1600s were used and whatever the next big thing is at the time has been used ever since, but without success."
Matthew French, director of the Poop Company, which is carrying out the search, said the divers would be there for two months, working seven days a week.