For most terrestrial mammals, swimming involves walking in the water. In other words they use the same gait as they do on land. The more fully aquatic species are, however, more adapted to the water. Beavers fold their front legs under their chests while swimming. Moving towards more fully aquatic species, Sea Otters float quite happily on their back while seals are so adapted to the water that they are clumsy and foolish looking on land.
Different species swim in different ways.
Many seals and sea lions use both front and hind limbs when swimming, but Elephant seals only ever use their hind limbs. In the Cetaceans which are fully aquatic, the hind limbs have been so adapted to become the tail flukes - a single member. This 'tail' has the flukes horizontal rather than vertical like the tail of a fish. Others will toboggan on snow. Finally, some mammals have taken to the air.