Ultrasonic sensors are so cheap I just replace them once a year or so, but I’ve not for a few new prototypes going with ToF sensors and one using a pressure sensor, to look for a less wasteful solution.

Vegetronix water level sensors sensors: https://www.vegetronix.com/Products/AquaPlumb/

They also have good soil moisture sensors that IIRC work via time domain reflectometry which is more accurate and lasts longer in the field.

There are black "waterproof" (weatherproof?) ultrasonic sensors that last a way longer time

I've heard that pressure sensors are the most reliable.

There are magnetic sensors in which a floating magnet (sealed in plastic) position is read by sensors (Reed, Hall, etc) sealed as well.

There are new 60ghz sensors available that can do this (they can see through walls so you could have the sensor completely enclosed (maybe even potted in epoxy?!)). Sparkfun/Acconeer A121/XM125 is what I'm using,although not in this context, it's for my robot.