It depends on positive or negative gradient but mostly it doesn't matter in DW.
More important is combination SSP TYPE bottom limited with type of bottom because mud or sand decreases sensors effectiveness (sand only 25% mud 50%)
Positive gradient curves acoustic wave up and negative down
(Diagrams above shows situation where source of pinging is close to surface)
But "bottom limited" is usually in shallow waters and differences are minimal
Too shallow depth will disturb your sonars - too close to surface waves noise
Too close to bottom is dangerous with higher speeds because turning could change your depth about 2-3 meters