We did the 2.5 hour kayak tour to the small island just off the coast of Kailua Beach. The check-in, as others have mentioned, is slow and disorganized, and probably takes at least an hour from the stated tour start time, which is a real disappointment. Also, although this is a guided tour, you will have to drive the kayaks to the beach yourself. I was miffed at this at first, until they explained that this was a requirement from the city to keep the beach from becoming too crowded. In any case, it is not really difficult to drive the kayaks.
Once we finally got in the water, we really enjoyed the leisurely paddle to the island, which is very close to Kailua Beach. The snorkeling was actually really good; the water was very calm and we saw a good variety of fish and coral, as well as a turtle and a white-tip shark. Be advised that the best snorkeling can be found if you swim a good bit away from the island itself. Our guide was very helpful and informative and explained a lot about the nature on the island and in the water.
Overall, this would be easily five stars, if only the experience had been a little more streamlined.