The warning was made by Dr Suwanchai Watanayinchai, director-general of the Disease Control Department.
Suwanchai said the first Saturday of March each year saw a national awareness campaign day to prevent children from drowning.
Statistics showed that one-third of drowned children died during the summer school holidays from March to May, Suwanchai said.
During the past 10 years, an average of 334 children under 15 drowned each year, he added.
In 2017, 708 children drowned, with 254 or 35.9 per cent of those deaths happening during the summer holidays, Suwanchai said. He added that most of the victims aged between 5 and 9.
He said the drowning deaths mostly occurred because children swam alone without the presence of their parents.
Last year’s statistics showed four incidents where three children drowned at the same time, and 10 incidents where two children drowned together. Suwanchai also noted that more drowning deaths happened in northeastern provinces.
He said most drowning deaths occurred between midday and 2.59pm.