Hi @nemog, welcome to the forum 
This comment and the thread that it’s in are likely worth a read - it seems to be on a similar issue.
Beyond that if it’s possible it may help to measure the actual speed of sound in the water at the time of testing (since it varies based on temperature and the water salinity), but I expect that would show up more as a reasonably consistent scaling error (multiplier of the measured distance), so if that’s not what you’re experiencing then the main issue is likely something else.