The 2011 Tōhoku tsunami destroyed several palm trees, sent sand into an ancient fishpond and deposited more sand into the Bay. Snorkelers with confidence swim over the rocky formations to the north and south of the Bay.
In the morning, this is a nice family spot, especially for children and adults who want to swim or float. Beach chairs are plentiful, provided by the adjacent Marriott. In the later afternoon, you'll likely see turtles resting if you walk along the lava formations south of the beach (past the Lava Lava club). A beautiful shoreline walk is to the north all the way through the Hilton grounds (stop to watch dolphins swim in the lagoon), then along their golf course onto a narrow, rocky path that leads to the remains of an ancient village. This last section requires shoes!
The ancient fish ponds adjacent to the beach are interesting to explore.
Near the main parking lot, the outdoor shower and changing room/restroom is clean. Stop at the Island Gourmet Market at the Queen's Marketplace for lunch supplies. They have a wide variety of good sandwiches, sushi and lunch plates ready-to-go or made-to-order.
Another convenient access is through the public parking lot next to the Kolea condo development. A nice path leads between Kolea and the Marriott to the north end of "A" Bay. Just south of the Hilton parking lot is another public parking area, with a path through the lava to the shore trail. It leads to a spectacular sunset viewing area. If you're staying at a nearby Waikoloa condo, this is a handy path.
Good description and directions to "A" Bay here http://www.kona123.com/abay.html
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.