Fish have a streamlined body and a tail which allows the fish to swim well. Fins are important too since fish are adapted to live in water. Fish have a sense organ called the lateral line which is used to detect movement and vibration in the surrounding water.
Because they have gills and they use this for swimming and they become good swimmers because of their gills. They are also used to swim under water. And the last but not the least answer is that God made them like that, God made each species and organisms unique from each other. Ok? Did I helped you??? Thats all! =p