We went to these islands 10/2004. Antigua had the best snorkling of the 3. I don't have any information on the Sting ray excursion in Antigua, but this is what we thought of the 3 islands. St. Lucia the water was cloudy. Barbados the water was not crystal clear either, but we really enjoyed the excursion that we took.
We took Silver Moon cat. snorkling excursion. We snorkled with turtles and the Silver Moon is first class.
We took a snorkling excursion in Antigua. Although we did not enjoy the excursion itself, Antigua had better snorkling.
We did not snokle in St. Lucia. We did go to the beach and the water was not very clear.
Hope this helps.