We tend to use a lot of ultrasonic devices for tank levels. You simply configure the device for depth instead of distance. Dave is right in that there will be a blind spot (blanking distance) where the device is switching from transmit to receive and this is normally around the 300mm mark but on some units might be as low as 150mm.
Personally I would go down the pressure transmitter route in this application. 1 bar = 10m of water so go with a low range unit for accuracy.
Here in the UK you wouldn’t need drinking water approval if it’s for watering plants and there is an air break between supply and maximum water level.