I fully realize that I am posting this opinion at my own peril but why would you want a system for determining your existing fuel level that requires you to go outside the boat? Sure, sight gauges are really nice and I have them but they require you to go (in most cases) into the ER. Mine are located through the lazarette hatch but still a pain to check when
you are underway.
Tank Tenders are easy to install and can reside in almost any location that is above the tops of the fuel tanks. No electricity is required and they are powered by Mother Nature. (Barometric Pressure)
The outside dial reads inches of diesel while the inside dial reads inches of water. On my boat The Tank Tender agrees 100% with the markings on the sight gauges. (I only use the sight gauges to confirm that the Tank Tender is working properly. (ie: no leaks, breaks in the tubing, etc.)
Right below the Tank Tender is a label that indicates how many inches of fuel or water are in the tank & those readings are converted into gallons.
Super simple and no power is required.