Some sea animals breathe underwater, and others just hold their breath a really long time. Whales and dolphins and other mammals (and birds too!) are in this category. They just stock up on air before going underwater. Fish and amphibians (like tadpoles) breathe with gills that take air out of the water. When water flows and churns, air mixes into it, and fish can breathe this air out of the water. If you have pet fish, there's always water running somewhere in the tank because if you let it get perfectly still, there wouldn't be enough air mixed in and the fish would struggle to breathe. Gills are very interesting organs, and need to stay wet to work, so fish can't actually breathe out of water.