The most important thing - The Manual!
They always say RT*M - so here it is. Your boat may be slightly different. If you have a factory manual with changes, send it in along with your hull number. Thank you!
It is important to know where water may be coming in. This is the drawing I modified for my P-35. I laminated it and put it on the head door so it is readily available to guests. Right Click on image to enlarge. Note that I changed the head to freshwater, and now the old head intake is used for A/C intake instead.
Your layout may be different.