An electric motor and electronics are only as good as their power supply. In this group you will find LiFePO4 marine batteries and chargers designed for them. They are suitable for powering both an electric motor and a fishfinder.
A LiFePO4 battery, or lithium iron phosphate battery, is much lighter than a traditional lead-acid battery, which you notice in the weight of the boat and when carrying batteries to and from the car. It handles deep discharge well, and the voltage stays steady for almost the whole discharge. That means an electric motor keeps its performance well into the day.
Size the battery capacity around the consumption of your motor and electronics and the length of your fishing day. Many anglers run a separate battery for the fishfinder, so electrical interference from the motor does not show up on the screen.
A LiFePO4 battery needs a charger designed for it, as a charger made for lead-acid batteries will not charge it correctly. Store the battery over winter in a cool, dry place with a partial charge, and check connections and cables before the new season starts.