I've spent the last hour looking for this story on Google, and it's broken me, so I humbly ask for help.
What I remember:
Sam and Dean were out on the ocean working a case. I may be mixing this up with another story completely, but the the thing they were hunting was warded off by Lima beans (some kind of beans) instead of salt. Towards the end it knocks Sam silly, and Dean's alone fighting it off and it knocks him overboard. Sam summons some kind of ancient warrior to help him defeat it in the end.
There also may have been a man on a dolphin offering to help Dean once he'd been in the ocean for a bit.
Uh, anyone know what I'm talking about?
Edit: Found! It's
Occasional Demons by
marinarusalka.