Some other P42 owners may chime in, but I am not sure that you will find any details about your fresh water system. On my 1991, I asked Hunter and there were none.
There is a fresh water tank under the vberth, and one to port under the floor which is under the water heater, which is under the settee. On my boat there was an A-B switch and small trash filter under the floor of the storage cabinet between the nav station and the aft end of the port settee. Some have this valve under the settee or under the water heater.
The main water pump was under the base of the companionway stairs.
For lines it gets worse. Mine were all the gray RV crap, and ran to the starboard of the center line of the boat and was a rats nest in getting to and from fixtures. The check valve for the city water inlet was under the fridge and other than "seeing" it little could be done to replace or service it.
Similar for the shower heads and worse for the stern washdown/shower. Access on these boats is pretty tough, especially for the aft head, as the only access is from the engine compartment and then only if you take a bunch of stuff out.....
Shame on manufacturers for this style of installation (most of the others are similar) for something that needs service or replacement. IMHO.
All the best, and if there is something specific you are looking for, there are a couple of 42CC owners active here that are very helpful.