All the oil and gas platforms I’ve been to have devices colloquially known as watermakers. They are desalination units that convert seawater into potable water. They usual work by reverse osmosis, as I assume thermal desalinators use too much power. Examples of watermakers are by C’Treat and Village Marine.
The commercial ones you see installed in front of shops in Malaysia work on the same principles, though they don’t work under the same conditions (they only filter good old M’sian tap water).
You know the offshore watermaker isn’t working correctly when water tastes brackish.