Why Do Fish Need Water?

They need water because they have gills, not lungs. Mammals lungs are designed to enable them to extract oxygen from the air, whereas fishes gills only enable them to extract oxygen from water. There are a few exceptions of fish that have both gills and lungs, but mainly that's why they need water.

Fish need water because fish get there oxygen from it. It is similar to us breathing air which is what we get our oxygen from . Fish have an organ called gills that water passes through and they absorb the oxygen from it with these gills . All fish have them and that is why they need water to live.

They are "designed" to use their gills to get oxygen from water. It is a special way they do this. It is evolutionary. Before humans are born, they have gill slits, like fish. But they eventually loose the need to use them and are born without them.

Their bodies function to extract oxygen from water (through gills) rather than from air (through lungs).

Becuse the have gills no lungs

Why did you answer your question in your question if you knew the answer already?
 
That made your question kind of like a fish out of water here, didn't it?

Cause fish got to swim, birds got to fly,and if God wanted frogs not to bump their rear end on the ground he would have given them wings...in other words they we're designed that way.....but there are exception to every animal/species....such as walking catfish/mud puppies,birds-penguins that fly under water so to speak but are flightless out of water, and frogs that jump from tree to tree over long long distances sort of flying(gliding on the air).....en.wikipedia.org