Got it. I know for sure that the beach by the townhouse and also the beach by Mauna Kea have very good snorkeling. I don't remember the beaches south of Kona. I recommend you buy that blue book that I recommended and look carefully at where snorkeling beaches are and where sandy beaches are. From the description of the house it sounds like you can snorkel near the house but there won't be any sand -- only rocks. That is a usual situation on Big Island and that is why I recommended that you stay north of Kona. Sandy beaches are located there. Also I would not stay on the main road. It would be way too noisy for me.
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I just remembered that our friends stayed here and really liked it:
http://www.2papayas.com/reef_house/
That agency http://www.2papayas.com/ has more houses and condos besides that house and most of properties are on the beach and not on the road.