The Collaborative International Dictionary of English v.0.48:

Dogwatch \Dog"watch`\, n. (Naut.)
   A half watch; a watch of two hours, of which there are two,
   the first dogwatch from 4 to 6 o'clock, p. m., and the second
   dogwatch from 6 to 8 o'clock, p. m. --Totten.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dogwatch
    n 1: either of two short watches: from 4-6 pm or 6-8 pm