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tdwhitehorn,
The plumbing pipe and fixtures used on your whaler are probably not compatible with PEX fittings you can but at the local hardware store. That is because of the additional stress and pressure put on marine plumbing systems. There are a few different manufacturers of this type of marine / rv plastic plumbing and I do not know what brand your is, but like Phil said, you can contact Whaler directly for that info. You will also not be able to shut off just one line to the hot water tank as there is a inbound and outbound water line to the tank and from my experience just shutting one line off will not allow your presurized system to work independently of the hot water tank. However, you should be able go to West Marine or online and purchase a Camco hot water tank winterization and bypass kit. They cost around $25 bucks or so and has all the hardware and fittings needed including a valve that will bypass the tank and allow your water system to operate independently of the hot water tank. It is also a great tool to use to winterize your hot water system without pumping an additional 5-10 gallons of antifreeze into your system to fill the hot water tank so the antifreeze will properly be distributed thru the hot water plumbing of your boat.

Jim G.