How fish and other animal get the oxygen inside the sea?

The aquatic plants (seaweed, duckweed, elodea, ext.) create oxygen. That is why fish can live in ponds, lakes and oceans. Often in fish tanks have a bubble blower, or the fish will swim to the surface of the water. If you are thinking about rivers, depending on how many rocks and how fast the river is, if the river is fast with a lot of rocks at the surface of the water, the rocks will stir up the water and create bubble fields the fish can live with.