Houseboat Safety Improvement Act of 2003 - Directs the Commandant of the Coast Guard to prescribe rules for houseboat generator exhaust safety that: (1) set a standard that ensures that carbon monoxide from such exhaust is not expelled in a manner that would impair the safety of houseboat occupants or swimmers near the houseboat; (2) require a houseboat manufactured after a date specified by the Commandant to meet such safety standard; (3) require houseboat manufacturers that do not comply with such standard to issue a recall for such a houseboat to retrofit it to comply with the standard; and (4) require any person who commercially rents a houseboat, or commercially sells a used houseboat to another person, to inform such person of the potential hazards of carbon monoxide poisoning from houseboat generator exhaust (particularly for houseboat occupants or nearby swimmers), if the houseboat was manufactured before the date set for compliance with the safety standard under this Act and has not been retrofitted to comply with it.