The tourist party had disembarked just hours before the luxury Indian Ocean Explorer and its seven-man crew, all from the Seychelles, were seized.
The 115ft long vessel is a former oceanographic research ship converted for dive trips popular with well-heeled British and American tourists.
Cruises between seven and 12 days start at £2,000 per person aboard the seven-bedroom ship.
“We are thankful that our guests were not affected, but we are of course very concerned for the welfare of the crew,” said Lynda Teasdale of Aquatours, a London-based firm which arranges dive trips on the boat.
“There is very little information about where they are going. We can only hope that this is over as soon as is possible.” The attack took place 600 miles west of Mahe, the main island in the Seychelles archipelago, in a cluster of coral atolls called Aldabra, a World Heritage Site.
“The vast majority of tourists come to the inner island resorts but a small number are more intrepid and visit the outer islands,” said Fergus Cochrane-Dyet, Britain’s High Commissioner to the Seychelles.
“I believe this yacht was used for visiting Aldabra. What I’ve been told is that they believe it is being taken to the Somali coast.” An international naval force, including ships from the Royal Navy, has been patrolling the waters close to Somalia since January, after more than 100 vessels were hijacked there last year.
This has significantly reduced the number of successful boardings in the Gulf of Aden.
In response, the pirates are venturing much deeper into the Indian Ocean. The Aldabra islands lie more than 700 miles south-east of the Somali coast.
“We are seeing more and more attacks in the region of the Comores, Seychelles, the islands of the Indian Ocean,” said a diplomatic source in Nairobi familiar with the pirate crisis.
“It’s clear that the international force further north is stopping attacks there, but these guys are just shipping out to the south and finding more targets there. It shows the scale of the job we have in trying to stop them at sea.” Pirates hold ships for an average of three months while ransoms are negotiated.