Got a Garmin 5xxxS (551, I think, but they're all pretty much the same) attached to a dual frequency Garmin sounder fitted by RC in the transom well. Worked perfectly for three years.
Been out yesterday and today... at low water depth, mostly 10ft or less but occasionally 15ft, the depth reading starts to flash. Sometimes, the reading - whilst still flashing - varies, suggesting it's still reading an accurate depth; at other times it seems to freeze at some 10-15ft figure. Once I hit deep water the problem eventually clears. Back to shallow water and it flashes again.
When not flashing the depth shown appears to correlate with charts.
The Garmin handbook doesn't talk about flashing depth readings.