I installed one of these watercounters in my boat about 6 months ago and it seems to be reasonably accurate. Has a totalizer on the sending unit, and a digital readout that you can remotely mount anywhere in the boat and reset.
I use a sight gauge. Since the water level in the sight gauge tube tube will remain constant and level with the tank water level anywhere in the boat, the tube doesn't have to be plumbed directly into the tank and can be placed in any convenient location. I used an unused tee near the water tank outlet but ran a clear plastic 3/8"hose into the lazarette where it is attached to the bulkhead right next to the hatch where it's easy to see.
I put cheap brass ball valve in the end of the tube as the pump sucks air through the tube when the water level is low.
I also have a Watercounter which I installed about 1 year ago. It works very well. I installed the unit in the engine room where there was a straight run of water pipe just upstream from my water pump. It is easy to install the sender.
I put the gauge in the panel near the wheel where it is easy to see. The gauge can be put anywhere in the boat.
In my boat, the water tanks are positioned so I cannot use a stick to sound them. I ran a line from my Hart Tank Tender, an item I like a lot, to the water tank. I can sound the 2 fuel tanks, the holding tank and the water tank and I am done. Simple, not too expensive and effective.
But I know how to make it into an audible audible alarm since it is a real bother to walk all the way to the stern and to lean out to see my chines. If Stretch added an aft cabin, he could hear the water slapping under the chine as he slept!
'73 GB 42 CL #362
Punta Gorda, Florida