The reason why sharks can swim fast is that unlike most fish, sharks have tails that are not horizontally symmetrical . Instead , the top of a shark's tail protrudes further back than the bottom , giving the shark lift while swimming and making it "spring" forward at speed.
Most fish make a single water "ring" that moves backward at the end of their tail's stroke but sharks are thought to produce two rings because of their asymmetrical tail which bespeaks a more efficient propelling action.
It has also been discovered that sharks can raise the tiny teeth-like structures called placoid scales on their skin to create tiny wells across the surface of their skin which reduces drag just like the dimples on a golf ball, thus enabling the shark move forward faster.