Did you check the side of the case for a date? Some have tags there. It could be something else but lead acid batteries seem to have a predictable life span of around a 5 year average(according to many that keep track of these things). I'd guess the batteries had a year or few on them when you bought the boat Gene?

My test is just to charge and then let them rest for a few days. The voltage will begin to drop, especially compared to the newer batteries, the battery is history. When I can find a date, it's usually 5 years and maybe one or two more.

I buy mine at a local auto parts store. I found though our local Hamilton marine had a better deal on starting batteries this season. Check around.