However if you have a detector in a different pressure and salinity layer to the source the detected frequency will be altered - doppler shifted if you will - by up to 10% either side of nominal.
Not so.
The frequency of sound transmitted by the ULB may be impacted by the water pressure etc. in which the transducer operates. However, for a stationary transmitter-receiver (hydrophone) pair, the frequency received will be identical to that transmitted.
Slight frequency variations could, of course, occur if the effective transmission path length through the ocean water between transmitter and hydrophone changes due to currents, changes in temperature, salinity, pressure etc.