The idea behind HHO is to use the alternator of a vehicle to provide electricity to break the bonds of water molecules to make hydrogen and oxygen, which according to the claims, will add free additional energy to add power, give higher fuel mileage etc. Over the years, the theory changes as to why and how this is supposed to work. Unfortunately, the physics is that the energy released during combining oxygen and hydrogen to make water is the same amount of energy that is required to break water into it's basic elements in a 100 percent efficient process. We can't come close to 100 percent efficiency in using electrical energy to break down water, and current combustion engine and generator systems are very wasteful as well so this is far from even being a break even process.
As for the sidelines of water in the engines, etc., they're just part of the fun of internet forums.