I cannot recommend this trip enough-I have made numerous trips to the Hawaiian islands, and I have done a lot of boat tours and shore dive/snorkels, and this is one of the best things I've ever done (It is 'must-do' along the same lines as the Kona night manta ray snorkel and swimming with dolphins in open ocean in Kona).
They take you (a small, personal, boat with only a handful of people) out a few miles, where there are pelagic fish and sandbar sharks (and occasionally others). Then, the safety diver (ocean conservationist and researcher Ocean Ramsey) scopes things out, and you get to snorkel with these amazing, beautiful, peaceful creatures. I stayed in the water for about an hour, but you could really swim for a shorter amount of time if you wanted or needed. They provide interesting education on the sharks, their importance, types of sharks, and their behavior on the way out (they had just gotten back from giving presentations at UCLA on shark behavior- they know what they are talking about). These observations were spot-on, and the experience was amazing. They are very passionate about the ocean and shark conservation, and protecting this vital part of the ecosystem. They do often see dolphins (and whales in-season) but since we told them we had already swam with dolphins and were more interested in sharks they focused in this.
You might be thinking that swimming with pelagic fish and sharks in open ocean would be terrifying, and that you would rather be in a cage- but it really is not scary at all. My wife was so afraid of sharks she would ruin our snorkeling trips (just the thought of sharks was too much for her), but she got in and interacted with them with no problems. This is not just for thrill seekers (though admittedly I do feel like I got much more of an experience out of this than being in a cage for a few minutes). This is for anyone who really wants to get the experience of being out in the open water (safely) with sharks, but also wants to learn more about their behavior, and support a great organization that is trying to save our oceans. It's also perfect for people who are scared of sharks. There is nothing else anywhere like this.
Ocean and Juan, the owners/operators are extremely friendly, helpful, and are experts at keeping the encounters safe and personal. Their goal is clearly to help you have a life-altering experience that will change the way you see the ocean and sharks (and of course you can tell your friends you went diving with sharks in open ocean- and leave out the part that it was a calm, non-stressful, really blissful experience).
Do this! You will not regret it. Plus, Juan Oliphant is also a renowned ocean photographer, and if you have sone extra cash, he'll take professional photos of your memorable encounters. Don't waste the money on an impersonal boat full of tourists getting dumped into the water (I've done several of those- and wish I had the money back to do this OneOcean trip again).
They also offer scuba certification and freediving training, which, given their friendliness and level of ocean expertise, I certainly would not hesitate to do with them as well.