Gold
The premium currency in the game is gold and it is earned by doing missions, quests and unlocking daily chests (there are five daily chests per level per day). To earn gold from the missions you must complete all three of the missions you are given. Try to extend your survival time for as long as possible as doing so will increase the amount of gold you receive from the Survival Bonus at the end. Spend the gold you earn to upgrade your shark.
Gems
Gems can be used to buy premium items and upgrades for your shark. Always keep an eye out and eat fish that have a purple glow as these fish have gems. You will have to have a full boost to catch them as they move fast and are not easy to catch.