I'd like to say thank you for the very kind words regarding my efforts aboard HAWSEPIPER. I'm grateful for them and I'm humbled beyond measure.
To respond to the water tank question. I got the tank from WholesaleMarine.com. It's a 12 gallon water tank. My wife and I are just like you, we bring gallon jugs of water aboard to drink and cook with. The water in the tanks is just for washing hands or dishes. Additionally, the reason I added the pump is that I extended the waterline to the aft cockpit and installed a quarter turn hose bib there (under the shelf where the flip up seat is on the starboard side). This lets us rinse the mud and sand off our feet after beachcombing with a short garden hose so we don't track it all over the boat.
With regards to draining the system, the tank is easily removed for winter storage. Two straps hold it down, 3 hose clamps (easily accessible) come off and the tank is out. I will then use LP air to blow the lines dry.
By the way, if anyone wants my old water tank (and helm seat), come and get them, no charge.
Respectfully,
Jeff