Is a search of a houseboat conducted without a search warrant illegal?

Albers claims that the entire houseboat search was illegal because the rangers did not first obtain a search warrant. The law is well settled that "absent exigent circumstances, a warrantless entry to search for ... contraband is unconstitutional even when ... there is probable cause to believe that incriminating evidence will be found within." Payton v. New York, 445 U.S. 573, 587-88, 100 S.Ct. 1371, 1381, 63 L.Ed.2d 639 (1980). But there are exceptions. The government asserts-and the district court held-that the lack of a warrant did not render the search illegal because houseboats are covered by the vehicle exception to the warrant requirement.