The Samui Harbour Office on Wednesday warned the ferry operator to be careful in both directions on the Surat Thani run as waves as high as three metres were spotted in the Gulf of Thailand.
The office also prohibited small boats from leaving the island’s shores.
The ferry boat operator was warned to check lifejackets and all safety facilities on their boats.
The Meteorological Department warned that the monsoon trough on Wednesday and Thursday will lie across the upper North and the upper Northeast due to the active low pressure over the South China Sea, while the strong southwest monsoon will prevail across the Andaman Sea, and east through the Gulf of Thailand.
The department warned that strong winds and 2-3 metre high waves are forecast in the Andaman Sea, along with waves of about two metres high in the upper Gulf, and above three metres in areas experiencing thundershowers.
All ships should proceed with caution, and small boats should stay ashore, the department warned.