Simrad IS20 Manual page 60

Depth
The default value for the depth offset is 0.0, which
indicates the displayed depth from the transducer to the
seabed (b). Refer to the illustration on next page.
The value should be increased or decreased, depending
on whether the depth reading should be from the water
line or from the keel respectively:
60 | Configuration
− A negative offset equal to the vertical distance
from the transducer to the keel will display the
depth as measured from the vessel's keel (a)
− A positive offset equal to the vertical distance
from the transducer to the water line will display
the depth as measured from the water line (c)