I was part of a big group sailing this feb, and we were supposed to leave a day or two after that big blizzard hit up north. If something happens and you truly can't get there, this is what several people had to do: meet the boat at its next stop. You miss the first day or two of your cruise, but it is still better than missing all of it. We picked up two groups at our first stop this february. The biggest thing for that is to keep in contact with the cruise line so they know what is happening.
Obviously, it is much better to be able to get on the boat on time, but just in case, there are still some other options.