The Collaborative International Dictionary of English v.0.48:

Mackinaw \Mack"i*naw\, Mackinaw boat \Mack"i*naw boat\
   A flat-bottomed boat with a pointed prow and square stern,
   using oars or sails or both, used esp. on the upper Great
   Lakes and their tributaries.
   [Webster 1913 Suppl.] Mackinaw

WordNet (r) 3.0 (2006):

Mackinaw boat
    n 1: a flat-bottomed boat used on upper Great Lakes [syn:
         mackinaw, Mackinaw boat]