Ship Graveyard shortcut
After the gigantic fall after the tunnel there will be sharks in the water and a waterfall on the right if 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.
Far East Shortcut
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.
Lost Island waterfall secret
Near the beginning of Lost Island you will see a waterfall. You can go through it and it will take you through a small tunnel where you can grab a blue boost and a red boost as well as get way ahead of the other boats in the race. Later on in the course you will see another waterfall. You can't go through this one but you can go up it. At the top of the waterfall you will find a small passage with a red boost at the end. WARNING unless you are way way ahead of the other boats I would not go up this waterfall without a boost tank that is at least half full because it takes a fair amount of time and it isn't a shortcut, in fact you lose time but if you have a boost tank that is at least half full the red boost should fill your tank and you can boost for just about the rest of the race if you hit all the other remaining boosts which will more than make up for the time lost going up the waterfall.
Press and hold the A button right after the announcer says "one!" (but before he says "go") and you'll get a "Super Start."
Venice Canals Shortcut
When you get to the part with the three hard turns in a row (left, right, left) you can skip all of this by jumping over the island in between them both. This comes shortly after the part with the docks.