I have a Westerly Discus with a stain- less steel, fresh-water tank situated under the bunks in the forward cabin. Although I have wanted to find a way of monitoring the water level, this has been difficult as the tank is irregularly shaped and particularly inaccessible. A simple and cheap solution has been to set up a manometer measuring tube in the under-cockpit, stern-tube access locker as shown in the photograph.
The cold water feed hose was cut and a T-piece with a 5mm side arm inserted (as designed for garden irrigation systems and easily available on Ebay). A clear plastic tube was used as the vertical arm together with an on-off valve (arrowed). It is important to turn this valve off when the water pump is in operation to prevent air being sucked into the system. The “empty” point can be marked as shown.
This solution has worked well during the past season although care is needed when the water tank is filled to the brim to prevent the manometer from overflowing.