This has to be one of the best snorkeling sites on the Big Island. There's lots of marine life and it's not as crowded 'cept when the boats bring tours.
It's difficult to get to Cook's Monument. You can't drive there. Your options are kayaking (what we chose), a boat tour (costly, limited time, crowds), a horseback tour (intriguing but limited time), or a long hike (easy downhill - a killer up).
Kayaking was relatively easy, and safe. You're in a bay, so you're not going to get swept out to open ocean. When we started the wind was coming in, making for a longer trip to the monument (about an hour), but still not dangerous at all. The return trip was quicker and the waves carried us in. Our 13 year old easily handled a single kayak on his own. The rest of us paired up. It was our first kayaking experience of any sort, so don't be put off by inexperience. Many folks there are first timers. It's a great place to learn.
When renting a kayak, the folks at the shop strap it on—and can fit them on ANY car—including multiple kayaks. They give you instructions and directions, then it's off to the bay. At the launch site, locals are around to get you all set up. PLEASE tip them if you use them. They do this for a living, are excellent at it, and are well worth the tip money. They help you in, and out, and replace the kayaks on your vehicle too. Go early for the best parking (and a long time enjoying your day).
We were fortunate to have spinner dolphins all around, often swimming within feet of our kayak. They swam to us—it's illegal to paddle to them. It's a nature lover's treat to have such luck.
For landing, do NOT land at the monument itself... look where others have landed and join them. It's a great picnic spot and an easy land path to the monument for a closer look and a trip to a foreign land (Britain owns the land the monument is on).
Snorkeling is gorgeous and easy for all ages. It's helpful to stay away from the boat tours when they arrive... crowds... but they don't stay long. Look closely and you can see eels lurking in the rocks deeper down. Underwater cameras are awesome here. We have a special case for our Canon digital—well worth the money for the great pics.
It's best to wear reef shoes, wet suits, and gloves. This water is more shallow and you don't want to step (or touch) urchins. NOTE: Wet suits do NOT protect you from urchins, but they do help you stay warm in cold water so you can enjoy yourself for hours while others need to get out due to cold...