to defeat the first two bosses place your boat at the lower edge of the screen a bit right from middle (near monsters) and just shoot rapidly.
When you cross the finish line, spin in the air for 1,000 points per rotation.
At the beginning of any race press the B (fire) button and you'll get 1,000 or 2,000 points.
In the first area, go through until you find a 1-up, get everything you can in the level, but don't cross the finish line. When you run out of energy and die you'll go back to the beginning of the level. The 1-up and everything in the level will be back. Do this as many times as you want, since you won't lose lives because you can continue getting the 1-up.
Before a boss fight starts, press the B (fire) button constantly, That way you can take away 1 or 2 energy bars away from the boss.