The 400 include some 20 divers and crewmen from Phuket who had to be plucked from the sea earlier by another tour boat when their own boat, the liveaboard Little Princess, was overwhelmed by heavy waves and sank at about 6:30pm.
Before it sank the skipper managed to put out a distress call and the crew and passengers were picked up by a speed boat waiting at Koh Tachai. Rather than brave the trip back to te mainland, however, the speedboat went back to the island.
Because the HTMS Pattani cannot come close inshore the 400 are being transferred by navy inflatable boat and the operation is expected to take some time.
The Phuket News understands there have been no casualties of any kind.
Once all its passeners are aboard the HTMS Pattani will take them to the Royal Thai Navy Third Fleet base in Tab Lamu.