9 second boost
At the start of the race, between countdown from 3 to 2 rev your boat up about halfway, then let go to let it go back down. When it is all the way back right at 1 rev it up again and hold it until the race starts. If you did it you will start with a nine second boost. It may take some practice, but it works for all boats.
Infinite turbo
Pause the game and press A, B, Z(2), R, L, Up, C-Down, C-Left, Right, Z.
Shortcuts
Far East - After the Blue Boost that's hanging out in the air, you'll hit a bunch of small waterfalls followed by a Tunnel. If you jump on top of the Tunnel, you'll cut you're time in half for that section.
Ship Graveyard - After the gigantic fall after the tunnel there will be sharks in the water and a waterfall on the right. If you take the shortcut there will be 1 red boost and 1 blue boost and you can use boost to boost into 6th or 5th depending on where you were before the waterfall.
Venice Canals - Towards the beginning of the level, after the second checkpoint, there is a wall with a red arrow pointing left. Usually you would turn a sharp left, then a sharp right (a full 180 degrees) slowing you down and causing you to bump into the walls. But instead of turning that sharp left, just hydro jump over the wall. It will cut about 6 or 7 seconds off of your time and you will end up in 4th or 5th place.
Venice Canals 2 - Towards the beginning of this stage there is a wall that has poles coming out of the water and there is a board going across the top of it. Do a hydro jump off that wall and to cut off a lot of time from your race.
9 second boost...