Water depth performance is how far away (in multiples of the depth of water below the transducer), depth information can be measured. THIS IS THE KEY FOR SHALLOW WATER OPERATION.
Detection Range is how far away a target of a certain size can be DETECTED. But detection is different from localization.
FarSounder can accurately tell you the depth of a target consistently for at least 8 water depths, (in practice we have many data sets of 12-18 water depth performance for some bottom types and conditions). Beyond FarSounder's water depth performance capability, if a big target is detected, range and bearing to the target can be calculated and displayed. However, the depth of the target is not neccessarily known.
Read this tech blog posting to learn more about the physics behind water depth performance.
Just as RADAR does not give altitude above the water but can still be a useful tool, detection at beyond water depth capabilities can also be a useful tool.