There are several posts on Water Wetter ... no one has been able to come up with independent testing to support their claim.
Some believe that WW appears to lower temps just a very tiny bit and it is likely associated with WW specifying a higher water to coolant ration, hence just a tiny bit more efficient at cooling - higher water content equals cooler temp, all other things being equal.
BTW, I lived in Florida - never did find the fountain of youth. Don't think there is magic in this line of effort, either.
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg