Just a few minutes' walk from Kabushima is Marient, a small but appealing 'Marine Science Center'. There are plenty of educational displays in Japanese, which look like they're about tectonics and oceanography, but the main attraction is the aquarium part.
The tanks house a diverse range of marine life such as clownfish, starfish and tiny jellyfish; there's also a touch tank where the kids can get to handle some of the residents, and a small tank full of 'Doctor Fish' which happily nibble the dead skin off your hands, which isn't bad for a mere ¥300 admission. Upstairs there's a roof terrace and a restaurant, both with fine views out over the ocean and back to Kabushima. For more information (in Japanese only), the website is here.