During the early '90s I assisted Red Line Synthetic Oil Corporation in its development of the "streetable" formula of Water Wetter. The *original* Water Wetter product came in crystaline form and was intended for racing engines and not street use. It was not until about 1993 that the street version was perfected.
I've used the current Water Wetter product for many years in a number of different types of engines.
As stated by others in this thread, the product works by modifying the surface tension of the coolant to which it's added.
The active ingrediants of Water Wetter are 1) a surfactant, 2) a lubricant for water pump shaft seals and 3) a corrosion inhibiter similar in nature to what's in GM/Texaco "Dexcool".
That Red Line Water Wetter (RLWW) has a very effective inhibiter package means, when added to water, you can run a "no-antifreeze" coolant and have the same cooling system durability you'd get with a normal 50/50 mix of water and antifreeze.
As for Water Wetter's effectiveness in reducing coolant temperatures, well---it varies with cooling system design, capacity, coolant mix, system pressure and nominal coolant temperature.
Where RLWW has maximum effectiveness is in a cooling system where some level of nucleate boiling is present. Often this will occur in systems that are running near their boiling points. It also may be most likely to occur in areas of the cooling jackets that aren't getting as much flow as other areas.
In engines that have little or no nucleate boiling and are running at coolant temperatures significantly below the system boiling point, RLWW may have limited impact on coolant temps.
A lot of folks who road race small-block V8 powered Corvettes with stock or near stock cooling systems in hot weather, seem to have great sucess with reduced coolant temperatures after adding Red Line Water Wetter.
Conversely, there's no doubt in my mind that other cooling systems, especially those with high flow rates and nominal coolant temperatures quite a bit below their system boiling points, may not see as much reduction in temperature after the addition of Red Line Water Wetter.
In many cases, the harder the engine is run, the closer to the system boiling point the coolant temperature is and the more nucleate boiling is present, the more effective RLWW will be.
Water Wetter's other attraction is that, in areas where freeze protection is unnecessary, it can be used to replace the corrosion inhibiters and water pump lubricants present in antifreeze and allow a 100% water coolant.
I've done this for a number of years in both my Corvettes, my Chevy Malibu and my daily driver 2001 Camaro.
Those who misunderstand how a cooling system works will argue that using no antifreeze will cause boil over but, in reality, with a 15 lb pressure cap that works properly, a Corvette cooling system will boil at 257 deg. F. With a 50/50 mix of antifreezer and water, it will boil at 264. While antifreeze gives you 7 more degrees, going to straight water, a more efficient coolant than a 50/50 mix reduces coolant temperature such that the seven degrees is, IMO, irrlevant.
During the development work I did for Fluidyne High-Performance Division that brought the Fluidyne 90-96 C4 Radiator to market in 1998, both of our hot weather test cars used straight water/RLWW coolant mixes. One tested all summer in Tucson AZ and the other tested during August in Palm Springs, California. There were no boilovers or even "near-overheating" incidents---even during near-racing speed tests, uphill on mountain roads in 108 deg. ambient temperature. That says much about the Fluidyne radiator and 100% water/RLWW coolant.
Now, I'd like to address a couple of statements made elsewhere in this thread.
When you put an additive in your radiator, and the water temperature runs cooler, that means that the coolant is not absorbing as much heat.
A preposterous statement that is nothing but incorrect.
As for corrossion (sic), use nothing but distilled water and water pump lube in climates where you do not need antifreeze Corrosion comes from the water and all water has minerals that corrode .
While straight distilled water and a water pump lubricant will work well in a race engine, don't try it in a street engine. Corrosion of all metal exposed to distilled water will set-in very quickly once coolant flow ceases when the engine is shut off. It is the metal that corrodes and that will take place whether minerals (in tap water) are present or not. Coolants made up of water and water pump lube work well on the race track but not on the street. Coolants used in street engines must have a corrosion inhibiter.
The.problem with factory systems is the dont run enough pressure . If you can run 20 to 24# caps you'll see better results too.
It is true that some cooling systems can see their performance enhanced by operating at a higher pressure, however, you want to be careful when going to a 20 or 24 pound system in a production engine using production hose clamps. In many cases, stock hose clamps and even replacement worm-gear clamps may not be reliable on production-type coolant hoses used with 20-24 psi systems. Additionally, many radiators that use an aluminum core and plastic tanks sealed by "o-rings" may not be reliable at 20-24 psi. This is not a reason to discount higher pressure systems but it is a reason to re-examine how the hoses attach to cooling system components and to consider your radiator construction, if you decide to raise your system pressure. In most racing applicatiohns that use 20-24psi systems, you'll see braided-stainless-steel covered hoses attached with AN fittings to rugged, all-aluminum radiators.
Oh well , like anything else ,ya either like it or ya dont. Stricking revelation huh? 
