Very easy. I installed a 23 gallon plastimo water tank. Plumbed it up to a pressure pump with a integral pressure switch, so that when the pressure drops, the pump comes on, like when you open a spigot. I replaced both sink faucets on the boat. You will need to wire the pump and provide a fuse or breaker and a on/off switch. I got all the parts from Defender. Probably 400 bucks for everything. Get the faucets at local big box hardware store and all the hose and fittings too. I do not use it for drinking, so I did not use food grade hose or install any filtration. On my Oday 25 with the outboard, I installed the tank on the floor under the cockpit between the stringers and on a rubber mat where the inboard engine would have been. I installed the pump a few inches away from the tank outlet and below the tank so gravity can prime the pump. On the pump discharge I put a tee in the line so I could feed both faucets. Works very well. Put an on/off switch and always leave it off when away from your boat. If a leak develops the pump will start and eventually when the tank is dry, the pump will soon be ruined.
