To the original posters question, there are many quality tank gauge products on the market. Some do not require anything inside the tank but rather a sensor that mounts or attaches to the outside of the tank.
On my previous boat, a Catalina 34, it didn't have a gauge but you could always when it was getting near and that was because it had a hand pump and it would get a little harder to pump of the tank was getting full.
On the current boat there is a gauge that came with the boat. Click the lever left and it reads the water tank, click it right and it reads holding tank. Friends bought a new Catalina 355 and launched this spring. It had 3 water tanks and 1 holding tank but no gauges. He bought a tank monitor system that involves a type of tape sensor that attaches to the outside of the tank. Once in you calibrate at full and empty. His has sensors for all 4 tanks.
On the other debate of to go or not to go, my theory is that I spent nearly $200,000 on my boat, I spend thousands each year on slips, storage and other costs and if I want to use the dang toilet I'm going to. I'm not nor would I let any family or guests be miserable just because I didn't want the toilet used. Especially at anchor when getting off the boat ain't all that easy. A pump out in out area is $6 and is probably the cheapest thing on the boat. So even if we pump too frequently, it's not a big deal. Now we do use common sense and try to put as little paper in it as possible to avoid problems. But as Stu pointed out the stuff breaks down. Especially after a day where you have some chop.
