For those not living in Thailand, the sea is very warm.
The average sea temperature in the Andaman Sea is 28c to 32c (82f to 89f).
This water temperature is considered to be very warm and does not give any pleasure from swimming.
If you think it is very hot in Thailand and plan to cool off by swimming in the sea, a better idea would be to swim in your resort pool.
It is worth taking special care of the possibility of infection with various microbes in this warm water.