It's a perfect date spot, but you are sure to have a great time with just about anyone, be it family or friends. The aquarium is huge and really well built, not too far from Osaka City center.
I recommend going there in the afternoon, and buying the Aquarium + Ferris Wheel combination ticket. When you're done with the aquarium it will probably already be dark, which means you'll have a great view of the city lights from the huge Ferris wheel! Good amusement/shopping complex nearby.