CALL it the Cruise Ship Coverup. The dirty secret behind the outbreak of stomach illness on 21 cruises this year is that the cause is other cruise ships in the narrow Caribbean shipping lanes. The big ships routinely dump raw sewage into the ocean. Ships following behind then draw in the polluted sea water for their desalinization plants, which take out the salt, but not all the bacteria. Passengers are soon suffering from diarrhea and vomiting - even if they are careful and drink only bottled water - because they are showering in sewage and brushing their teeth with it. "But no one wants the public to know because it's just too disgusting," one nautical expert told us.