The aquarium is not on the island, it's just before the bridge.
I think it's not so expensive because you will see some stuff really cool there...
The major good point is the big water tank. There is a lot of fishes and also a shark and a ray. There's a show but it's all in Japanese. It's mostly for kids and it aims to lear child about the fish names. It still fun.
You have also a big part for jellyfish. I loved it. You can see it on the pics on this page.
There's a show with dolphins but it was to crowded so I couldn't see that much except that you should not sit on the first row due to the splashes.
You have a lot of other cool things to do. You can touch little shark, see big turtles...
To conclude, you will spend a very pleasant moment. After this you can go on the island and enjoy the sight.