I just replaced a 2 and a half year old battery for a similar issue. Battery was purchased 6 months before I got the boat but my depth finder kept turning off if my rpms weren’t up. Started watching the volt gauge and the volts were dropping below 12 if I wasn’t going at least 5 mph. Read where humminbirds need at least 12 volts.
Was shocked with the battery being less than 3 years old, but I got a new battery and problem solved.