Getting your dog familiar with water such as the sea is very important and you should introduce them at a very young age if possible.
Swimming pools can seem like a daunting thing for a dog as there may be no sloped edge for them to easily get out of.
Try to entice your dog to start going for swims in lakes, river banks and the sea where there is a gentle slope going into the water and where the dog can easily get out of and feel safe while in the water.
Teaching a dog to swim is very important as they could some day be in a situation that involves water and it could be life threatening.