My take on what water wetter does is based on what I learned from wrenching on diesel generator sets and from Caterpillar's research.
Years ago I got called to a machine in a hospital for water in the oil. It turned out to be the liner seals had failed and water from the cooling system leaked past them into the crank case. Upon disassembly I found terrible erosion on the thrust side of the liners on the cooling system side. In my opinion, they were a short time away from becoming porous. This engine had straight water in it without additives. I tried to get them to pay for new liners but they refused so when I reassembled the engine, I turned all of the liners 90 degrees so to put a "new" surface on the thrust side. I also put a water/antifreeze mixture in the cooling system.
This is where Cat comes into this... My dad was from Cat. Factory trained, etc... He was able to somehow show me a VHS of X Ray films made by Caterpillar of a cylinder wall of a running engine. One with only water and one with Cat's water additive. The plain water was being pushed away from the liner as the piston pushed down. It was similar to watching waves at the beach. The second part had water additive. It adhered to the inner surfaces of the engine much better than plain water. Also has rust inhibitors.
Ever since then (probably 35 years ago) I have never run straight water in any engine except for doing a flush. In NY we have to run antifreeze for winter but I have always used either the Cat additive or Redline in the rare case we were not using antifreeze.
They need to stop saying it lowers temperatures. It's probably the easiest to understand sentence they came up with. It definitely makes the system more efficient.
These are MY findings. Feel free to tell me right or wrong.