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.
As of May 30, 2023, Riptide GP2 is switching to a free to play, ad-supported model. The free to play version includes a Premium Mode which turns off ads and returns the game to its previous Premium state.