To answer a likely quick question, the finish line buoys DO count. I did not unlock the achievement until I managed a run hitting those last buoys.
I found this quite easy with a Novice boat (I used "Damn the Torpedoes"). Start up the race, getting a boost start is not necessary, but helps. Remember to collect boost. After the first drop, go straight through the waterfall, and collect the green gate unlock. Grab the red boost, and aim to the left; Buoy #1 will be here after the drop. Align without the boost, and make sure to hit it.
When you get to cavern with the wooden structures, go straight, and jump through the wooden barriers. Buoy #2 will be shortly after; go around the ramp and aim for it for an easy hit.
When you get to the large open area with the large passageway to the left and the open waterfall to the right, go left. Buoy #3 is at the end of the straightaway you hit after the small drop. Again, aim right for it, and it's an easy hit. By this point, you should have the lead (comfortably most likely), and you won't have to worry about boats catching you.
Buoy #4 is after the downhill after the section requiring several short curving turns, at the start of the large open area with waves. I hit the one to left, but don't hang too far to the left, or you will end up in a small passageway on the left.
All that's left is the finish, which again DOES count. Just ram it, it's a very clear shot.
Hopefully that works for you. Not all the shortcuts I mentioned may be required, but they really gave me a comfortable margin so I could hit the buoys and not worry lost momentum, so I recommend them.