A fish pushes itself through the water by curving its body from side to side and swishing its powerful tail fin.  The other fins help it to keep its balance and help guide it, and for braking.

Most fish have swim bladders filed with air inside their bodies.  The swim bladder acts as a float so that the fish can lie quietly in the water without rising or sinking.

A fish can control its depth in the water in much the same way a submarine varies its depth.

To go down, a fish lets some of the air out of its swim bladder.  To go up, it forces more air into it.

Fish called “darters” have no swim bladders. They have to flap their chest fins to keep afloat in the water.  Nature has shaped the fish so that it scarcely disturb the water as it swims.

Man, seeing  how easily and swiftly fist cut the water, patterned his boats and submarines after a fish’s streamlined body. – Dick Rogers