The Collaborative International Dictionary of English v.0.48:

Midships \Mid"ships`\, adv. [For amidships.] (Naut.)
   In the middle of a ship; -- properly amidships.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Amidships \A*mid"ships\, adv. (Naut.)
   In the middle of a ship, with regard to her length, and
   sometimes also her breadth. --Totten.
   [1913 Webster] Amidst

WordNet (r) 3.0 (2006):

amidships
    adv 1: at or near or toward the center of a ship; "in the late
           19th century, engines were placed in front, amidships,
           and at the rear" [syn: amidships, amidship,
           midships]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "amidships":
   average, betwixt and between, central, core, equatorial,
   equidistant, half-and-half, halfway, in medias res, in the mean,
   in the middle, interior, intermediary, intermediate, mean, medial,
   medially, median, mediocre, mediterranean, medium, mediumly,
   mesial, mezzo, mezzo-mezzo, mid, middle, middlemost, middling,
   midland, midmost, midships, midway, nuclear