Water level detection, where water is somewhat polluted and has significant dissolved salts
Given the working parameters, the Ultrasonic Ranging solution indicates the best maintenance-free option. Depending on the desired MTTF, it need not be an expensive solution either.

The budget approach involves modules such as this one:

 (From eBay, for under $2)

Rather than deploying the module as-is, the module can be potted using epoxy potting compounds, covering everything but the tops of the ultrasound transmitter and receiver and the connector pins. Better yet, suitable weatherproof cabling can be connected, and the junction also potted. This promises reasonable longevity.

With a larger budget, other ultrasonic ranging modules can be found, which are IP67 rated, designed from the ground up for extreme environments.