You definitely see stories online about missed port weddings.
I knew someone in the same situation - cruise left out of Miami with wedding scheduled at first port (Key West). Her ship never went there due to mechanical problems shortly after setting sail. Even had she booked through the cruiseline, couples must still abide by laws where they marry, so the port they went to later in the Bahamas had laws that she could not meet on short notice (residency requirement or something like that). So it doesn't matter *who* you book through as each port has it's own laws and the coordinator can't change the laws of the port (blood tests, residency requirements, etc). She ended up marrying back in Miami that she put together while on the ship.
You also see many brides who plan their cruise weddings in the Caribbean smack-dab in the middle of hurricane season, then get very upset when their cruise is rerouted. Hurricane season technically runs June 1 - November 30, but inclement weather can happen any time. Some brides just ignore the hurricane season dates and schedule weddings then anyway, taking the risk.
Another factor to be wary of is if tendering is involved. Sometimes ships go to Grand Cayman but cannot tender in due to rough waters. Would be tough to have a wedding scheduled in Grand Cayman, be able to see the island, but not be able to tender in.
And as always, the fine print of the cruise contract states that itineraries can change at any time for no reason at all.
On a positive note, we had a port wedding while on a 7-night cruise in 2003 (St John, USVI), taking a total of 19 people (all cruise guests). It turned out to be everything we dreamed, but we also were aware of the many "what ifs" that can take place on a cruise. Having cruised previously, we knew of the risks but were willing to play the odds. It was a terrific feeling to set foot on St John that morning and even better to feel the sand between my toes as I walked down the aisle at 9:00. A reception onboard a chartered catamaran capped off the morning followed by a private tour of St Thomas for our group that afternoon. Everything worked like clockwork, even with five ships in port.
Definitely purchase "wedding insurance" or work with a coordinator who has some sort of guarantee as well. Our's would reschedule our wedding up to 12 months later if we missed port... just another reason to return to that beautiful island.
We were intent on marrying there, and were set to return had we missed port (even if it was just the two of us). Now after taking everyone there on a cruise, they all want to return!