In general, stick to the water as much as possible. You’ll travel faster, which is more or less the point in a racing game.
If you’re in first place and still have a bit of track ahead of you, it’s often a good idea to hit the brakes to allow the racer behind you to pass, allowing you to shoot them down and effectively remove them from the race.
Don’t forget that hitting the brakes while you’re in the air will cause you to drop suddenly. It’s a good idea if you’re dedicated to collecting items as it will help you get back to the water sooner.
Since new boats and upgrades use separate currencies, don’t be shy about upgrading your existing craft while you save up for a new ride. If you’re close to buying a new boat, you may want to hold onto some of the upgrade currency to use on it, but otherwise it’s usually better to make what you’ve got faster.
If at first you don’t succeed, upgrade your boat and try again. You have nothing to lose by repeating a race, and you’ll almost always gain. Your stats will eventually be improved enough to give you the edge you need.