I agree with booking a tour, however on our trip to costa maya we booked a personal tour (not thru the cruiseline). It was much cheaper and our guide was actually born and raised in a house in the midst of the ruins (the ruins were undiscovered at the time and later discovered by his father) so he had some great first hand knowledge of them.
October 1997 Princess (Bahamas)
October 2000 Carnival Tropicale
October 2006 Carnival Miracle
October 2009 Carnival Legend