I have to disagree with some of the advice given by PPs based on my experience:
My previous boat had a 9.9HP OB with alternator which charged my battery while the engine was running -- about 1 hour each day that I took the boat out, perhaps twice a week from May to October. That was the only charging it got. It was drained by cabin and nav lights, navigation electronics and VHF.
During the winter I removed it from the boat, kept it at home connected to a Guest 3-stage charger. The battery was 7 yrs old when I sold the boat last summer and still alive and healthy.
Before I bought the 3-stage charger I could never keep a battery alive for more than 2 years.
Given the light use the OP makes of his battery, I think it could last forever -- at least, from my experience.
Fluctuat nec mergitur