If your specimen is indeed a Mollusc, then I would agree with Geoff that
Teredinidae is your best bet.  However, keep in mind that many non-molluscan
marine worms (Phylum Annelida) also construct calcareous tubes, usually
attached to various solid substrates.  In the Philippines are some really
huge Teredinidae, of the genus Kuphus, which can exceed 1 meter in length
and 50 mm in diameter.  But I'm not aware of anything similar in the
Caribbean area.

Paul M.