Hmm, my colors were black and white as well but I didn't have a "theme" other than the color combo.
For favors there are lots of cute black and white favors available. Bottle openers, bottle stoppers, chocolate dipped strawberries, black and white cookies, mint tins, candles, black and white m&m's, etc.
As far as decorations, what were you thinking?
You can buy white starfish and seashells and sand. You can tie black ribbon around the white starfish and attach a note for favors. Or make a centerpiece out of the white sand with black candles...