In Hungry Shark Evolution, you play as a shark whose goal is simply to eat things (and people) and avoid starving, or avoid fish and other objects that damage you. There are two types of currency in this game. Coins are the primary currency and are used to buy most of what you need, but gems are the premium currency and are used to buy some real good stuff, such as unlocking sharks early or buying the Bloodbath power up. Read on to find out how to get more of both!
To get more gems, you can do one of two things. One of them is to find a gem fish, which is a fat green fish that only pops up every so often, in completely random places, although it usually tends to be in deeper, darker areas of the ocean, and the more advanced the shark, the more it tends to happen. You can also buy more gems at the store.
There are a number of ways to max out the coins. One of them is to stay alive for as long as you possibly can, to increase the survival bonus. Each minute that goes by causes the survival bonus to increase, and the more fish that you eat, the less fish that there will be for you to eat later (usually), so stretch out the eating of fish, people, pelicans, etc.
Additionally, there are various items that you can find and/or pick up along the stages, by searching along the bottom of the ocean and the bottom of various caves. There are 15 collection items per stage, each of which nets you 25 coins, and which you can pick up each time that you play. They are in the same locations each time, making them relatively easy to find time and time again. There is also the daily reward chest, which you can collect once per day, but which earns you hundreds of coins each time.
Finally, any edible creature (fish, crab, human, etc) with coins will be gold in color. Eat them to gain more coins, and eat enough of them in a short time to begin a gold rush, which temporarily turns ALL of the creatures in the water gold, allowing you to max out on coins for this duration.