Most cats are capable of swimming if they want to. Not every cat likes water, but certain breeds of cats are adept at swimming, such as the Turkish Van.
Cats are natural swimmers, but because domestic cats don't encounter water very often, they are not used to it. A lot of cats may have had bad experiences with water, like rain or water used as punishment, which may cause some cats to shun water. As big cats like tigers and lions are able to swim in their natural environment, so can most domestic cats. Cats were also used to hunt fish in ancient Egypt.