Boatyards/Marinas are notorious for killing batteries. Most banks don't make it more than two years unless the owner is on top of it. I'm no electrician, but I doubt the guys taking care of this boat are either.
-Owner leaves 12v reefer and other appliances
on when he leaves. Either he doesn't get the shore power connected right or a mechanic/customer with a dock
cart knocks the plug
loose. Batteries get a nice deep discharge before anyone realizes what happened.
-Boat get hauled for winter and batteries sit, possibly with a small load and in cold weather
, for weeks before they get pulled from the boat and put in battery storage
-No one, owner or yard, checks the water levels. After one or two of these deep discharge cycles, the top of the plates are dry.
-The battery charger get confused by one thing or another and toasts the batteries while no one is watching. Or, due to too deep a discharge (see previous), gets confused and won't come on.
Again, not an electrician, but most of the battery banks I see killed are murdered through sheer carelessness, nothing complex about it.