We bought dive lights and swam out from the Westin/Maua Kea. Its very fun and low key. Go an hour or so before sunset and have dinner at one of the hotels. Even better would be too spend the afternoon at Hapuna Beach and end it with dinner and the rays. It might be a little scary if you are not comfortable in the dark ocean at night. The Mauna Kea Hotel turns on a spot light every night that will guide you exactly where you need to be. You may encounter paying tour groups out there, but that just adds more light to the area. The other two manta viewing areas off the Kona coast really require a boat.