Kona Bali Kai is a nice place if you want something simple and quaint. They are condos just south of the main stretch of town. Beach is right there. My personal fav is the Sheraton at Keauhou Bay. Nice place, amazing pool and views. Right off the hotel is one of the spots they do the manta dives. You can go out there and often see the mantas in the water around 8 o'clock. It is on the far south side of town though and there is no real beach there. But that is one of the nice things about Kona, you can pull your car over virtually anywhere and jump in to at least snorkel. If you are set on a beach to snorkel and boat diving, the King Kamehameha Hotel is a good option. On the north end, 5 minutes from the harbor and has one of the best snorkeling beaches on the island at it's back door. As for the manta dive, definitely do it! Don't worry about it being a night dive. You will be in the water with several dozens, if not hundreds of others (not as bad as it sounds). Everybody with a light. It's not a true night dive. Like driving down the Vegas strip at night. Most ops offer a two tank dive for that, the first being a twilight dive. Many times that is the better dive!