Depends where encountered. Rivers or lakes: 1d8 river boats (1d2 * 10 pirates each); Coastal waters: 1d6 small galleys (1d3+1 * 10 pirates each); Any: 1d4 longships (1d3+2 * 10 pirates each); Ocean: 1d3 small warships (1d5+3 * 10 pirates each). (See Water Vessels for details on ships.)