Officials from Tambon Koh Mak Administrative Organisation and local villagers tried to put out the fire, but failed as flames engulfed the boats before they capsized.
Officials said no witnesses had seen which boat the fire had been sparked in, but the blaze spread quickly to the six others docked side by side at Thong Lang Bay.
Damages have been estimated at 20 million baht.
Koh Mak mayor Nol Suwatjananon said the sea was rough with strong winds and high waves, so the speed boats had to be docked. Nobody was injured in the fire.
He said he reported the fire to the Trat governor and sought help from the Royal Thai Navy to help ferry tourists back to Trat mainland. The tourists were on the island for a day trip and the remaining six speed boats were not enough to transport them.