Chop fresh clams (if using), reserving juice; set clams aside. Strain clam juice to remove bits of shell. (Or drain canned clams, reserving juice.) Add enough water to the reserved clam juice to equal 2-1/2 cups.
In a large saucepan bring the clam liquid to boiling. Stir in sweet potatoes, onion, celery, sweet pepper, dried thyme (if using), and garlic. Return to boiling; reduce heat. Simmer, covered, about 10 minutes or until sweet potatoes are tender.
Mash mixture slightly with a potato masher. Stir in clams, fresh thyme (if using), tomatoes, lime juice, and, if desired, rum. Return to boiling; reduce heat. Simmer, uncovered, for 1 to 2 minutes more.