Looks like hammer time to me...I assume that is a cross and the formation surrounding it is extremely hard? I'd tap around on it with an appropriate blunt object and see how much of a fight it puts up. If you can't chip or break it off, the tumbler is probably the next shot.
I'm thinking it's a cross also... probably silver but hopefully gold.. I've never had any salt water finds to clean.. maybe lemon juice or vinegar will work