Simply put, manifold pressure is a measure of how hard the engine is gulping down the fuel/air mix supplied to it.
If you're used to driving cars or riding motorbikes, you're used to measuring developed power by looking at the rev counter. But in reality, that only tells you how much power the engine allowed to produce while weighed down by transmission gearing and vehicle weight, it doesn't tell you how hard the engine is "trying".
Think of it like this: You can see how much effort a runner is making by both seeing how fast his legs move, or by seeing how hard he is panting. Both are vailid, but they tell you different aspects.
As for the water charge, it may seem funny that you can improve a fire (fuel combustion) by throwing water on it, but it works like this:
The water is introduced as a very finely divided mist. This water mist sucks all the heat out of the incoming fuel/air charge (heat that was introduced by the supercharger compressing it to squeeze it into the engine); and, as hot things grow and cold things shrink, this means thecold fuel/air shrinks in volume, so you can squeeze more fuel/air into the engine.
And....
You don't really want a BANG! inside an engine, you want a burn. The more you can slow down that burn,the longer the expanding forces have to push down (or up, if you're German) on the piston head. Think of it like this: Which actually moves you physicaly accross the room, you're little sister's right upper cut, or your big brothers heavy push to the chest?
So it is with engines, and the water mist helped transfom the Bang! of the fuel/air explosion into the push of a controlled burn.
