Dead batteries from storage

Rookie move...forgot to switch the batteries off before storing for the winter. Pulled the boat out to check on it and make sure everything was good and both batteries are completely dead. I tried hooking up to the onboard charger. Nothing. Pulled the batteries out and hooked up to charger. Got a "bad battery" reading. I think they are both totally drained. A quick google search tells me that they are bricks now. Any chance of reviving them before I spend $$ on 2 new batteries? Thanks guys.