Loch Fyne extends 40 miles (65km) from the Sound of Bute and is the longest sea loch in Scotland.
It is a popular diving location and known for its oysters and seafood.
Keith Waugh, from a local diving club, said it was usually “incredibly safe” and many divers use the area as a training site.
He said the water near St Catherines, known as Seal Reef, has a reef near the surface and a small speedboat at the bottom.
Waugh, who has been diving for decades, said: “We usually explore the reef and see what fish might be there, and zig-zag to the surface.
“The reef starts really close to the surface so it’s ideal as a training experience.”
A Police Scotland spokesperson said: “The deaths are being treated as unexplained and a post mortem examination will take place in due course.”