how much variation in water level is there?
Could you mount a laser or ultrasonic distance sensor above the water
have a look at Pololu proximity-sensors-and-range-finders
for example
the VL53L3CX TOF sensor has a maximum range of 5mretres
the Maxbotix XL-MaxSonar-EZ2 Sonar Range Finder MB1220 ultrasonic sensors has a maximum range of 7.65 metres
I have used a JSN-SR04T waterproof ultrasonic sensor (maximum range 4.5metres) to measure river level height uploading data over LoRaWAN