Purpura patula is an intertidal species of rocky shores.  It is probably
quite rare in Florida for two reasons.  First, Florida is the northern fringe
of its geographic range.  It is really a Caribbean species.  And secondly,
the relative lack of rocky habitat in Florida.  I found them quite commonly
(some years back) in Cuba, and also found a few specimens in Puerto Rico.
Regards,
Paul M.