Is it a Humminbird? If so, welcome to the club. I have had my boat for five years. Right after I got it, I bought an 898C SI combo. I have basically had to switch out the thru-hull puck every year. It simply stops reading the depth. It does the same thing yours is. The puck is not that expensive bit on my boat it is a major operation to switch out. In order to get to the bilge, I have to remove three trolling motor batteries, the starter battery, a 3-bank charger, a single bank charger, and the metal flooring. Then I have to route the new puck's cables to the console. After all that, I have to lout it all back. This usually takes me about 6 hours.
What is so aggravating is that there is no reason for it to go out. There is no damage. It just stops working.
2001 Triton Tx-21/225 Mercury EFI