If you start out from the dock all good and then notice a few hours later during cruising with all the electronic bells and whistles on like ais and autopilot with the radar , then the depth finder starts to go out / flash read below.
Mine did the same thing , until I changed out the batteries/ added solar and added 1 more battery to the bank . I found that with all the devices on at the same time the first thing that would happen when I didn't have enough juice would be the depth finder flashing/ going out with the alarm from my Raymarine ( yours is prob a garmin) . it would make me furious when it would happen . But when I turned off all the excess equip like radar and such the alarm and the depth finder would start to come back . I found that I needed to add another battery , I also added a solar panel to help with the juice.
Also as a side note , my kids and wife had all their electronics iphones . ipads ( 12 devices in total ) as well as mine all charging with the radio on . drove me nuts trying to figure out where all the Power was goin :lol: