I was most excited about this activity during my short trip to Kauai recently. I don't think I did enough research into what I should bring, but a beach towel was the thing I overlooked and needed to buy one from Kauai Sea Tours for $25. You are given a dry bag to hold the things you want to keep dry. I suggest brining a few snacks if you can. They provide many cold drinks so don't worry about that. I had a waterproof case for my phone and kept my phone out the entire time to capture many photos.
Onto the tour itself... Breathtaking. Incredible views of Kauai. Our captains were Nono and Collin and they shared many stories and many facts about the island, safety, sea life, folklore and just general good advice. They were full of silly puns and quick whips. They seriously made this experience an incredible one.
The caves we visited were out of this world, they pulled up and into some. They pulled up to many pods of Spinner dolphins and right up to waterfalls. They warned us that the water may be murky, as it was. It was not the best snorkel, but the ride there more than made up for it.
Now, the ride back. I had a great time, but I have to admit it was right at my limit in terms of physical strength and general will. It was very rough and bouncy and I needed to grip the raft rope with both hands for dear life or I would have been knocked off. I am in my thirties and in good shape, but I was sore for days after that boat ride. For a span of about 25 minutes I had water constantly splashing my face and ears and I could not let go of the rope to wipe it off or I would have flown off even with my "feet belt." I am sharing this because I would have loved a heads up to bring ear plugs to spare my ears from getting flooded. Would I do this all again, IN A HEART BEAT and time and time again. Small price to pay for a view of a lifetime. All I can tell you is, DO IT! You won't regret it!