Switch the Batteries in the Boat, or Could Drain the Backup. This is a 2 -battery setup: Battery A runs the load 1 day, and recharges the next day. Battery B recharges and is used the next day, or is a backup if needed. A battery isolator or ACR keeps batteries separate except during charging.
WARNING: Do not just wire the 2 batteries together. If you connect two batteries directly in parallel without isolation, they’ll equalize. Meaning if one is low, it will pull power from the other, and you won’t know you are draining your backup battery.
You could get a manual selector switch, but you have to remember to switch. A diode battery isolator is cheap, simple, but slightly reduces charging voltage. And there are automatic charging relay smart chargers.