Beach
Ya Nui
Rawai, Thailand · sea 30 °C at its warmest, in May

Sea temperature month by month
Monthly means in °C, NOAA OISST v2.1 climatology 1991–2020, sampled 18 km offshore, at the nearest open water cell of a 0.25° grid.
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 Ya Nui?
The sea off Ya Nui averages 30 °C at its warmest, in May, and 28.4 °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 Ya Nui?
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 Ya Nui?
Ya Nui is on the coast of Rawai, Thailand, at 7.7679, 98.3063. The map on this site flies straight to it.