The Collaborative International Dictionary of English v.0.48:

Cay \Cay\, n.
   See Key, a ledge.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cay
    n 1: a coral reef off the southern coast of Florida [syn: key,
         cay, Florida key]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "cay":
   ait, archipelago, atoll, bar, continental island, coral head,
   coral island, coral reef, holm, insularity, island, island group,
   islandology, isle, islet, key, oceanic island, reef, sandbank,
   sandbar