i have a 525 hummingbird fish finder its about 4 years old, has worked great till now, just launched the boat for the season on the way to the dock i turned on the fish finder and its reading the bottom, displaying water temp but depth only reads 1.7 , 1.6, 2.0 then back to 1.7 when i know im in 15 feet of water,, any thoughts on whats wrong or how to fix it ???? thanks