Source : Webster's Revised Unabridged Dictionary (1913)

Coaster \Coast"er\, n.
   1. A vessel employed in sailing along a coast, or engaged in
      the coasting trade.

   2. One who sails near the shore.

Source : WordNet®

coaster
     n 1: a resident of a coastal area
     2: someone who coasts
     3: a covering (plate or mat) that protects the surface of a
        table (i.e., from the condensation on a cold glass or
        bottle)