Ian ... My Garmin 550s (transducer epoxyed in the transom well on centreline) works down to 2m. This is a replacement unit provided foc from Garmin a year ago. The original unit worked to 2m when new but after four years this suddenly shifted out to 4m for reasons not even Garmin could explain. They had it back, tested it, found nothing wrong but replaced it anyway. However, when questioned they told me that they couldn't actually bench test for shallow water performance.
You may have the same issue? Give Garmin Technical a call. They were very helpful and gave me a refurbished replacement (that looked new) even though mine was three years out of warranty.
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