Check out Shoretrips (.com), they offer some tours from a very respectible places. I can also vouch for their auto refund policy if your excursion or port visit is cancelled.
We were supposed to go in Jamaica, but didn't make it because of the hurricanes... no phone call necessary; our money was automatically returned to our account.
Empress of the Seas 4/21/06
Star Princess Singles Cruise 10/30/2005
Carnival Valor Singles Cruise 1/16/05
RCCL Mariner of the Seas 9/12/2004
Spirit of Chicago 2-hr Cruise 7/24/04
RCCL Monarch of the Seas 3/29/04
Costa Mediterranea 12/14/03
RCCL Sovereign of the Seas 9/21/03