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

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "dogwatch":
   anchor watch, bit, day shift, full time, graveyard shift,
   half time, lobster trick, night shift, overtime, part time, relay,
   shift, split schedule, split shift, stint, sunrise watch,
   swing shift, time, tour, tour of duty, trick, turn, turn of work,
   watch, work shift