The water tank in my P-323 is molded into the hull. It has since been converted to a holding tank - which I am not sure was the smartest thing to do.
Bear in mind that my boat is hull #2. They could have changed design later on to a free standing tank, but not sure.
I haven't owned the boat that long, nor have I really taken a close look. I assumed it was fiberglass molded. Next visit, I will take a closer look. The Port and Starboard water tanks appear to be the same, but again, I never looked that close.
Was down at the boat after posting yesterday and looked at the tank: it's not as I remembered. It's not held in by straps like the side tanks. It sits between low bulkheads forward and aft of the tank, and is held down by plywood strips about 4 in wide glassed to the hull sides. The fit of the tank athwartships is snug to the hull sides, as the tank is shaped to the same angle as the bow.