to spend you Coins and Gems, you can either visit the Evolve section, which is where you 'evolve' your shark to a different type, or in the general Store, which loads before a stage. There are numerous things to spend your currency on, with most of this coming in the form of levelling up your skills.
Be sure to make the most of what is on offer in the Store, as the Items can provide a valuable Boost during play, while things like the maps offer that extra edge while exploring the depths of the ocean.
Great White shark