what is the role of the sea and all its inhabitants? does the sea,with all its elements of life and challenges, represent or symbolize the nature of society in general, and Santiago's fishing village in particular?

I think the sea is part of the natural elements which feature so strongly in the novella. Hemingway spends a good deal of time drawing connections between Santiago and his natural environment: the fish, birds, and stars are all his brothers or friends, he has the heart of a turtle, eats turtle eggs for strength, drinks shark liver oil for health, etc. Also, apparently contradictory elements are repeatedly shown as aspects of one unified whole: the sea is both kind and cruel, feminine and masculine; the Portuguese man of war is beautiful but deadly; the mako shark is noble but cruel. The novella's premise of unity helps succor Santiago in the midst of his great tragedy. For Santiago, success and failure are two equal facets of the same existence. They are transitory forms which capriciously arrive and depart without affecting the underlying unity between himself and nature. As long as he focuses on this unity and sees himself as part of nature rather than as an external antagonist competing with it, he cannot be defeated by whatever misfortunes befall him.