If you've played Super Mario 64, you will definitely love this game!
Mario is on vacation with Princess Toadstool, Peach on Isle Delfino Island. As soon as they enter the island, Mario gets caught by the police and gets the blame for all the dirt all over the island. Mario and Peach didn't understand what happened, but then a mysterious Mario-looking guy shows up and kidnaps Peach...
Gameplay is the same as Super Mario 64, but this time Mario has a water jetpack! It makes a new platforming style with loads of options. In this game, you can even ride a Yoshi, change it's colors and there's a lot more to do.
A very solid 9.0. A must have for the GameCube and a great platformer. It should be around 20 bucks now, so go get it. It's worth it.