There's two main places you can use the mushrooms in water park: the first is across the sand in the underwater section, and the other is while gliding towards the start/finish line, using the extra speed to fly straight over Wario's kiosk.
The video below shows you a run that uses the second route, against a ghost that boosts across the sand. While using mushrooms while gliding wins, it is only by a narrow margin, so you may find it more beneficial to cut across the sand. There's not many routes to exploit in Water Park, so it'll be a case of good driving and making the most of boosts, as well as getting 10 coins as early as possible.
It's not possible to get a jump-boost off the stairs at the start of the lap, plus jumping will negatively affect your drift into the turn that immediately follows. If you position the drift around the next corner correctly, you'll be able to collect all three coins and straighten up to get a boost over the ramp too.