Sorry forgot the levels. It's a water reservoir for hydroponic applications. Basically clean water with ionic content (so it's pretty conductive), water depth from 0-100 cm (or a little more) would be a good range. Current air pressure I will know as well (using a BMP180) so can compensate for that.
Currently measuring it using an HC-SR04. Works quite well but sensor is not waterproof and needs rather careful installation, also needs to be calibrated with the actual depth of whatever container is being measured. A pressure sensor in contrast can be dunked in the water, sink to the bottom, and gives the real pressure. Waterproof versions of this same sensor cost about 10x as much and have a minimum distance of 20 cm which is way too much for my application (highest water level can be as little as 1-2 cm under the sensor, which is already pushing the limits of the HC-SR04).
I've looked into capacitive measuring - not suitable as it also requires careful installation, is not portable between different sized tanks, and will be influenced by the medium: as said it's water with nutrient solution, and this nutrient solution can vary greatly over time.