He said the body of Bhumi Jensen, 21 was found at the resort of Khao Lak in Phang Nga province about one hundred metres away from where he was last seen jet-skiing before the waves hit on Sunday.
Bhumi was the son of Princess Ubolratana and her estranged American husband Peter Jensen.
"It's a national tragedy," said Thaksin, adding that two men with the crown prince at the time were still missing, while his aide de camp survived.
Other casualties include former finance and commerce minister Borom Tantian, who was swept away by a 10-metre-high wave as he was getting out of his car outside the Palm Andaman Beach Resort in Phang Nga north of Phuket, said The Nation.
His daughter Kantawan Kulchanyawiwat, an MP for the Democrat party, suffered a broken arm while her youngest son, aged three, was also swept away.