i have an 85 cuddy, i added 2 pvc saddle tanks under the rod boxes in the gunnells. i used 2 pieces of 3" pvc about 4 or 5 feet long. i tied them together in the stern and put a pump with a built in pressure switch there.
this is a shot of the port side looking under the stern storage bin, the pvc tube coming up is the fill
this is a shot of the pump woth the connecting tubing coming from both sides
this is a shot of the starboard side
the biggest drawback to the way i did it is the capacity. i only hold about 6 gallons, but it is great for a quick rinse
and cleaning sunglasses. it would be nice to have 10 or 15 gallons.
this is the pump and the t that both sides tie into.