Personally, for anything over a year I'd look into selling them. You're in Washington
State, correct? Find an electric
car club around you, or if golfers are active put an ad up if they are compatible golf cart batteries.
Or if they are in better shape than the ones you have now, even marginally better shape, you can swap the batteries and then sell your old boat
batteries and keep the better ones.
Either way, old stuff breaks even when it hasn't been used much and a couple years is a bit of time spent tripping over some batteries while you are wondering if they will still be working when you need them.