Again, I agree with HD. Blo**dy hell, twice in less than an hour!
Lots of these on sale, all claiming to lower water temperatures by around 30 degrees C. As HD says, they break down the surface tension of the water, thereby reducing the size and number of air bubbles that attach themselves to the water jacket walls. This increases the surface area of water in contact with the water jacket, thereby increasing its thermal transfer performance.
However, I have seen test results that cast doubt on the manufacturers' claims.