The Google Play, Amazon and iOS versions of Riptide GP2 do allow you to purchase optional extra cash if you want to upgrade or unlock hydro jets faster in the game.
However, the IAPs are completely optional.  If you look at the versions that don't have IAPs (the Xbox One version, the Steam version, and the Humble Bundle version) you'll notice that the game tuning (how much you earn, how much things cost, how difficult it is) is exactly the same as the versions that do have IAPs.