Shells store the self expression and protective nature of our friends from the ocean. Everything housed inside is sensitive and adaptable, but the shell itself is the protector, the first line of defense against the evils of the world. Camouflage, visual mis-representation, and a greater cooperation between the mind and body is kind of how Shell works energetically speaking.
It can make you appear as you'd like, rather then as you are to others... Making it a great deal of help while you deal with internal issues that you don't wish to share. This same attribute can make it very helpful in promoting groups to work well together, helping everyone see what they need to see in each other so that everyone can get along and play nice.
Salt water shells can be very beneficial in the act of cleansing an area or a thought pattern. Salt helps to wash away, so a Shell from the sea also carries with it the inherent cleansing properties of the salt water it grew in, and this is one reason shells are very forgiving and nurturing.