Boat Stereo System

Unless something is wrong on your Bayliner, If you checked the battery with a volt meter before and after cranking the engine, You should find the outboard to have a alternator on it. You might want to install a  Perko style battery switch which would allow you recharge the stereo battery while running the motor and would keep the boat battery safe from discharge from the stereo..

If your getting a second battery, try to conceal it as much as possible since I have had batteries stolen before just having one.Volkswagen Golf

Alpine CDA-9851

Alpine MRD-M501

Alpine 12" Type R Ported

Alpine Guy is my hero

A Normal battery in its fully charged state will read around 12.5 volts with no load on it (lights, radio off). The output of your alternator should be around 13.8-14.2. You may need to raise the idle to check the voltage while the engine is running. A Perko battery switch allows you  to connect both batteries to a rotary style switch and then you can select battery 1 or battery 2 or both or off. If you normally run the boat for a season without recharging thr battery, i bet you will find  14 or so volt at the battery while it is running. Do a google search for Perko battery switch.