Hi babe1636, welcome to Cruise-Chat! Yes, a ship sailing through an oil slick could cause damage to the ship. So far, it has not affected the east coast and doubt that it would. I am sure they will be able to navigate around, through, or change itineraries to keep their ships sailing. If the ships don't sail, there is no revenue, if there is no revenue they can't pay bills, if they can't pay bills, well...you know the rest.
I understand your concern, but the cruise lines will do whatever they have to do to keep sailing.