Your sensor has a 4…20mA output which is perfect for long cables that might encounter electronic disturbances and “higher” resistance due to long distances.
You could just use an ESP8266 [ESPHome] (f.ex. Weemod D1) with an 4…20mA to voltage converter (like this one) and the attached sensor to measure an anlogue value that you just need to calibrate.
The calibration can directly be stored in the ESPHome config. So you have a WLan sensor that directly gives you Water level in m. You can also directly add the Temp sensor to it so you have an all-in-one sensor set 
More perfect would be the RS485 option, but I did not see any documentation on this on the sensor seller. If you need to reverse engineer this one its likely not worthy. With good documentation it will likely directly give you pressure values.