My wife and I did the sting ray excursion in March 2005. It was OK, but at least on the day we were there it was not as good as the string ray excursion that is offered in the Grand Caymans. Antigua was our least favorite stop so if you are going to go ashore the sting ray excursion is something to pass the time. The town looks great from the ship but looks can be misleading. The primary problem we had with the excursion was that the water was very sandy, that is, there was not a lot of visibility. The string rays are in a fenced area (along with a fair amount of other fish and other creatures). the fence may contribute to the lack of clarity since it will tend to hold in the sand.
We did an island tour in Antigua and enjoyed it very much - liked the island as a result too. Nelsons Dockyard is interesting, as are the forts on the high ground. If you can pick up a history-oriented tour, you may gain a bit better appreciation for the island. I agree that the town right by the docks is not very impressive. Much nicer farther away!