lightship

from WordNet (r) 3.0 (2006)

lightship
    n 1: a ship equipped like a lighthouse and anchored where a
         permanent lighthouse would be impracticable

from The Collaborative International Dictionary of English v.0.48

lightship \light"ship`\, Light-ship
\Light"-ship`\(l[imac]t"sh[i^]p`), n. (Naut.)
   A vessel equipped like a lighthouse, carrying at the masthead
   a brilliant light, and moored off a shoal or place of
   dangerous navigation where a permanent lighthouse would be
   impracticable, to serve as a guide for mariners; as, the
   Ambrose lightship off New York was rammed and damaged in 1950
   by the Santa Monica.
   [1913 Webster + WordNet 1.5]