There are no wreck dives that I know of on Kauai. I don't think any operators do shark dives either. You will see white tip reef sharks at avrious sites around the island. Turtles are easy to come by as well. One day at Sheraton Caverns I counted 20 on one dive. You might be able to make it over to Niihua, ussually around this time of year the ocean starts to become too rough to make the journey. But if you have the option to go, I suggest you take it. Niihua has the best diving for Kauai. There you can see Grey reef sharks, turtles, hawaiian monk seals, and if you are extremely lucky they do see the occasional whale shark, but that is very rare. Kauai is my favorite island among the hawaiian islands, and October is my favorite time to go. I ussually dive with Sea Sport divers, but there are other good operations on the island as well. I have heard good things about Fathom Five as well. Both shops are located in Koloa which is about 10 minutes away from where you are staying in Poipu.