How do dolphins “see” underwater?

As well as having extremely good eyesight (except the river dolphins who are bordering on being blind) that allows them to see both above and below the water, toothed whales use a sense called echolocation to navigate and hunt underwater. This means that they emit sound waves and then detect and interpret the echoes that bounce back off of other creatures and objects in the water around them, allowing them to build up a picture of their surroundings. 

More facts about whales and dolphins.

dolphin echolocation