Ferry captains from Phi Phi Islands to Krabi and Phuket say they see dolphin once a month, mainly from February until September. These dolphin sightings can be made going to Phuket or Krabi Provinces but mainly in low season and often are accompanied by bad weather from the Indian Ocean in the west. The Dolphins seem to come into the shallows during these periods. perhaps the feeding is better in these conditions.
Unfortunately the dolphins of Thailand are shy and never swim with humans. They will approach boats, ferries and sailing yachts, but will be gone the instant you to get in the sea with them. They were spotted in Tonsai bay in the late 1990s and sailing captains tried to get in the water with them but they were curious but not playful.