Beach
Haad Yao
Thailand · sea 30.3 °C at its warmest, in May

Sea temperature month by month
Monthly means in °C, NOAA OISST v2.1 climatology 1991–2020, sampled 15 km offshore, at the nearest open water cell of a 0.25° grid.
About
Beach in Ko Phangan, Thailand
Wikipedia has no article about this beach yet. It is on the atlas from its Wikidata record and its photograph.
Questions
How warm is the sea at Haad Yao?
The sea off Haad Yao averages 30.3 °C at its warmest, in May, and 27.7 °C at its coolest. These are monthly means from NOAA's OISST v2.1 satellite and buoy record for 1991 to 2020, sampled at the beach's own coordinates.
When can you swim at Haad Yao?
The sea reaches 20 °C or more from All year. Twenty degrees is where most swimmers stop calling water cold; below it, a wetsuit helps.
Where is Haad Yao?
Haad Yao is on the coast of Thailand, at 9.7748, 99.9672. The map on this site flies straight to it.