The snorkeling at Captain Cook monument involves some work! First, you have to get there. If you don't want to take a snorkeling tour, you have to rent a kayak. Then you paddle over there from Napopo - for us kayak novices, it took about 45 minutes to reach the monument. At the monument, there is only a tiny beach and mostly lava rocks. You need to find a place for your kayak, and once you are set up, you can start snorkeling.
I also recommend you explore the trail, which has some overgrown ruins. And don't forget to locate the plaque that marks the spot where Capt. Cook was murdered (the monument is not the actual spot).