Now that I have the full game, and have played quite a bit of it from the comfort of my own home (and on good equipment) I can say that pushing the paint aspect in the commercial is a big mistake because so far it has really only come into play on a limited basis on one specific area. Most of the time you use the water cannon to fight (knock an enemy over, then stomp them), or for accessing areas regular jumps won't.
There are also 4 nozzles with the water cannon. One is the main weapon, the other 3 are interchangeable.
1 The hover jet: the one you begin with
2 The Rocket boost: go really high.
3 The turbo charger: go really fast.
Most of this game is hardcore platforming and lot's of frustration over knowing what to do, but not quite being able to do it because they time everything out so there is very little margin for error in many cases (especially for the good secrets)
Let me also add that the water effects in this game obliterate anything else ever seen on a console on on a PC with one exception. The 3dMark2001SE Pixel shaders demos. (both the advanced, and the normal one.)
If anyone knows what I'm talking about, they remember how amazing the waves looked, and in the advanced demo how amazing the water looked with fish swimming beneath it. Well the Gamecube managed to get both effects working together, and I watched my best friend mess around with the water effects for half an hour when he came over yesterday. (he's a graphics whore in a big way) Either way it's a good game, and worth checking out if you have a Gamecube.