A yacht sank at Railay Bay, Ao Nang, Krabi, on Monday, about one kilometre offshore.
No one was injured or reported missing.
It is believed the vessal sank due to a leak while it was anchored offshore.
Only the mast could still be seen sticking out of the Railay Bay waters.
Worapot Lomli, the chief of Hat Noppharat Thara–Mu Ko Phi Phi National Park, said he received a report about the incident and ordered officers to head to the sunken yacht to investigate.
The vessel, which belongs to a local business and was anchored on Sunday night, was submerged in about five metres of water.
National Park Officers have put buoys around it to warn other boats.
The owner of the boat has been contacted to remove the vessel as it is causing a navigation hazard.