The game's really cool, but it's hard to make good TAS with such long and repetitive games. A year ago I tried to make one, but lost motivation somewhere at the beginning. But I have some notes to share:
In most of races (especially at the beginning and at the end of the game) it's faster to destroy green boat rather then rush forward. At the beginning your rivals use weak boats, so you can quickly tear them apart. After several levels they power up defense of their boats, and it may be faster to just win the race normally. But near the end of the game levels become way too long (while power ups stay fixed at the maximum), so again it's faster to crush your opponent.
Here's an example, very unoptimized. There should exist more clever tricks to deliver damage to green boat. And I did it without Memory Watch, thus it's only proof of concept.
Also, for quality run of the game it's essential to manipulate nitro drops (in backview mode) so that you'd have 9 nitro at the end of each lap.
I beleive it's possible to run the game in smth like 20 minutes, which is quite bearable for racing games. Well, maybe adding some artificial variety can keep it interesting for whole 25 minutes.