Water Wetter enhances the coupling of heat in the engine to the coolant. This is like eliminating the bubbles in the bottom of a pan of water you are trying to boil. Once the air bubbles are eliminated, the water will boil because the heat from the pan is coupled to the water.
Water Wetter doesn't *add* any water to the mix, it's there to aid in the transfer of heat to the coolant so it can be carried away to the radiator.
I don't know if I've ever seen any reduction in coolant temps by using Water Wetter but it appears to control the temps. My 300E seldom exceeds 100C even with stop and go driving in Phoenix in the summer. This is a brutal test of a cooling system.
Using 15W-50 Mobil 1 probably helps as well.