Optima Batteries are tough! I have not had them in my boat, but my partner has. He is sponsored by Complete Battery Source, an authorized Optima Battery Dealer. We actually had a load test done on a battery that had cracked (from bouncing out of a broken battery tray) and it load tested fine even without acid! They replaced it no questions asked too.
Call Complete Battery Source for Optima Batteries at (313) 532-3462, tell them Mini sent you.