From the Garmin customer support web site:
No depth/blinking depth in shallow water
To prevent losing depth readings in shallow water, check the following settings:
Check the frequency setting on the sonar device. If a dual frequency transducer is being used, the frequency may need to be set to Dual Frequency or 200 kHz when operating in shallow water. The 50 kHz frequency may be too strong in shallow waters.
Change the Range setting to Auto
Manually adjust the Gain setting. The Gain setting is the sensitivity of the transducer. If the device is having troubles reading in certain areas of shallow waters, try decreasing the Gain setting. This will decrease the way the transducer reads stronger return signals in shallow waters.
