The Kamogawa Sea World is a marine park for the whole family. They have a variety of ocean creatures available for viewing, including the famous killer whales (orcas), as well as beluga whales, dolphins, sharks and other smaller ocean creatures.
They also have a restaurant and a food court on site where you can take your family to eat lunch, as well as quite a few gift shops where you can purchase various Sea World souvenirs to take back home with you.
There are quite a few shows that you can watch outdoors, such as the killer whale show, the dolphin show, and others. Beware of the water splashing though, so either buy a waterproof poncho or sit high up on the seats.
If you are going there by car, you can get there in one hour or so by using the Umihotaru highway, if you are taking a bus or train it would take about two hours from Tokyo station.
I would highly recommend this place to anyone that wants to relax and see many creatures that you would not normally see if you were just swimming in the ocean. It is a lot of fun for the whole family.