This achievement is quite easy. You may not get it on your very first race on the Beginner series, but the way I did it, the first race I did I came 3rd, second attempt I came 2nd. I then upgraded my hydro-bike a bit, and finally came 1st.
This was on the very first race you can do in the game, NOT the tutorial. A few tips I can give you as I have raced jet-skis in real life. They're the closest comparison as rocket watercraft are fictional:
1. Having ridden jet-skis in real life, you need throttle to be able to steer. Use medium throttle (known as trolling speed in watersports) going into corners, and gently feather it. Don't use the brakes as they slow you down too much, easing off the accelerator slows you down.
This game is unrealistic in the fact you even have brakes, as real life watercraft don't. A jet-ski has reverse thrust, but you're not supposed to use it as a brake. This should also apply to rocket water-bikes like the ones in the game!
2. Drive as if you were racing a car round a circuit. Brake gently coming into the corner, and aim for apexes, feathering the throttle as in 1. above. Using brakes in a corner will cause you to go wide, and lose too much speed. The AI cheats in this game, until you can max out your water-bike you'll probably struggle.
3. Perform stunts early. Just as you hit the ramp, start your stunt. Waiting till you're in the air is too late, and will probably result in you crashing off your hydra bike as you won't have fully finished your stunt. You perform stunts by moving your two controller sticks in different directions.
4. Keep doing the same race over and over. If you're struggling this will earn you money and XP to earn skill points to "learn" stunts, and be able to upgrade your hydro, this will make it easier. Upgrading your hydro-bike will also earn you the Mechanic achievement for 15G's :)