We did this trip with Captain Spiros, Levante Speedboat Excursions www.zantelevante.com and would thoroughly recommend it. Because it is a speed boat, you only need half a day and you can get into the caves themselves.
The shipwreck is, well a shipwreck, I have to say not that exciting, although the cove is really nice and the water amazingly blue and the caves are pretty, but Spiros is the highlight, extremely entertaining and very knowledgable about the area. Watch out, he does have an eye for the ladies, but he is very funny and not at all sleazy.
We booked with Ian on the beach, but the trips are very popular, and also they won't run them if the wind is too strong, so book early in your holiday or you may be disappointed.
Whatever you do, don't book on the large boats. We saw several and they are packed in like sardines and you can't get near the caves. Everyone we spoke to who had done one of these trips said they were awful.