The centrepiece of Motobu's Ocean Expo Park (海洋博公園) is the Okinawa Churaumi Aquarium, which features the world's largest aquarium tank. The aquarium houses a wide variety of marine life, including whale sharks. Dolphins are also kept here, a practice widely acknowledged as harmful to the animals' health.
Unfortunately it's on every visitor's checklist, so it's usually packed. From Nago, buses 65, 66 and 70 run directly to the park (¥860, 50 minutes).