A manometer with a sufficiently long horizontal section should be able to amplify movements along its horizontal axis if one of the upward facing end tubes is smaller in diameter than the rest of the tube. Two air pressure sensors that might work to estimate the changes in water height would be MPX5010GP and MPXV5004DP. The MPXV5004DP has two pressure sensors that might serve to connect to both ends of the manometer. Not sure if this would be an advantage or disadvantage.