Go get a decent portable charger that will charge 10 or 12 amps and a digital voltmeter . Hook the charger up to the battery. If it charges up and shuts off then in all probability it's good. Unhook one side of the battery. A few days later hook the charger back up and see how long it takes to shut off again or show a full charge. If it takes only a few minutes to show a full charge again then it's good. If it charges for a long time that's not good. If it won't charge up and shutoff it's bad.

By the same token when your charger shuts off you should have something in the neighborhood of 12.6 to 12.7. You could have as high as 13.1 or so volts. But that will drop after a while its sort of a "surface" or "surplus charge" and the stabil voltage should be 12.6 to 12.7. If you check it again in a few days ( again with nothing hooked to one side ) it should still read 12.6 or so volts. If it doesn't it's not good.

Other than that see how it acts on the water. Some of the new electronic chargers will not charge a stone dead battery.

If you too deeply discharge a battery you can kill it....if you do it below freezing especially.

Never leave your battery in a state less than fully charged for any longer than you have too.

A small digital voltmeter, a decent portable charger and your set to take care of your batteries and figure out just about any problem.

Besides my on board chargers I have TWO portable chargers and a digital voltmeter. Total investment $250.00 dollars. I never have to Guess what the state of any of my batteries is.

One charger will charge a battery from a deeply discharged state. One will "optimize" a battery by charging and exercising the battery and help get rid of sulfation.

If I think I may have a problem I never trust a single device. I run all my batteries ( I have 6) through the optimization cycle at least quarterly.

You follow this advice you will always be in top shape on your batteries and you will know your in top shape and will rarely if ever need help again ,or get caught flat footed with partially or run down batteries again when you hit the water.

I got so sick of going fishing with other people who never properly maintained their batteries,too only get to the fishing hole to find out our battery was dead,or had to cut fishing short because battery was undercharged,or paddle back,get towed,or pull start a motor.

I swore I was going to figure out the best way to not have that happen to me if I could help it. Especially when I was traveling fishing and trip fishing.