Most fish are too heavy to float. Some sink to the ocean floor and live there. This helps them stay off the ground and up in the water. Still others, like sharks, must swim all the time to stay up, because they don't have a swim-bladder. It is difficult to swim through water.
Water is thicker than air.
They can easily swim around coral reefs. The fish living on the bottom of the ocean are flat - both top and bottom. This helps them glide along the bottom. Fast moving fish have special features. Besides being shaped like a torpedo they can pull in fins not in use to reduce drag.