The use of Water Wetter may not show any noticable difference in the displayed coolant temperature. Water Wetter is useful in eliminating hot spots in the engine by preventing bubbles from isolating the coolant from the hot metal.
I use Water Wetter because Frank King, the former Technical Editor of the Star magazine, recommended it. Frank had that job for decades and was a very smart man.
Water Wetter isn't a snake oil product like Slick 50. It comes from a company who probably produce the best commercial motor oil you can buy.
BTW, Water Wetter includes additives similar to coolant so that if you tend to run more than 50% water in the coolant mix, the addition of Water Wetter will offer additional protection for the water pump, etc.