My boat came fitted with a "tank full" light. Having retired from an industry where everything critical had a back up, I was worried about the lamp failing, si I fitted one of these.
https://www.mcsboatproducts.co.uk/our-products/wastewater
They are easy to fit, you provide them with the depth of the tank, then use a holesaw to cut a hole in the top of the tank and secure the sensor through the tank using a rubber grommet. Then cut a hole for the gauge itself (mine is in the toilet compartment but you could position it near the helm), then take a 12 volt supply to the gauge (I took mine from the toilet power supply so that it is on when the toilet is switched on )and add wiring between the gauge and sensor and the jobs a good un.