If you've been on the Autopia in Disneyland (called the Grand Circuit Raceway in Tokyo Disneyland) you can probably guess the basic idea behind Aquatopia without even seeing it: a mode of personal transportation, this time on water.
Where the Autopia gives you the ability to actually drive the vehicles (albeit with the "help" of a guide rail), you have no control whatsoever over Aquatopia's watercraft. While that takes some of the participatory fun out of it, it gives the vehicles the ability to take you on a bit of a wild ride. Your watercraft will twist, turn, backup, spin around in place, all while taking you on a seemingly random trip dodging jagged rocks, spinning whirlpools, and other watercraft. The direction your vehicle travels in is not random of course, but controlled by unseen technology borrowed from Tokyo Disneyland's Pooh's Hunny Hunt.
Although the watercraft look a little like bumper cars they never actually bump into anything. They also don't move very fast, and even through there are numerous twists and turns this attraction is suitable for kids and adults alike.
If you watch this attraction from the rail you'll see pretty much everything it has to offer; there are no surprises when you're actually on it. The vehicles only hold two or three people each, which means the lines tend to move slowly (there are two separate queue areas, which helps). If you're limited on time, I would recommend spending a few minutes watching this attraction instead of riding it. You'll probably have just as much fun.