The clearest snorkeling on Hawai'i can be found here at Kealakekua Bay. While you may rent a kayak to get to Kealakekua Bay, it is recommended that you go through one of the snorkeling boat companies (we used Sea Quest and were mightily impressed with their operation and our experience). The bay is just beautiful and the story behind Captain Cook's untimely demise is sad but, also an important part of Hawaiian history.
Snorkeling the bay just off the Cook monument is a thrilling experience. Note: much underwater life may be seen and visibility when we were there averaged 75-100' easily. The best time to go is in the morning ahead of the crowds arriving by every imaginable size of boat. Be careful that you understand that you MAY NOT climb up onto the memorial without permission as this area is a park and its use is restricted by pass only. The center of the bay drops off precipitously and there isn't much to see as you move towards the center.
Of the snorkeling we did on Hawai'i, this location is surely one of the most visibly memorable.More