Wow, what a fantastic trip. We followed the advice of some of the other people on here, and Wendy, I hope you will do the same. The best trip is with the Potamis brothers as these people said. They are situated right at the north of the island. Basically you go as far north as you can, then go along a small road down to the bottom of the cliffs.
It costs 15 euros to do both the blue caves and the shipwreck, and was well worth it. Because the boat is small, it goes right into the blue caves, and the helpers on the boat how how blue the water is by putting their hands in. They are also quite happy to take photographs for you. The boat then goes all the way round the other side of the island to the shipwreck, where it drops you off, (you do have to walk to the shore through the sea), and leaves, to pick you up 40 mins later. The water was amazing....the clearest and bluest I have ever seen, but was absolutely freezing, but very refreshing.
I guess the snorkelling was quite good, although I didnt take my set, but it might be worth taking a pair of flip flops or something to wear as the beach and sea is pebbley.
You cannot go to Zante without doing this trip!