There are two reasons why dogs drown in a pool.
First a dog that is heavy in the back area like Bulldogs, sink faster then other dogs. Not all bullies need a life vest but it is advisable.
Second, if a dog is treading water at the end of a pool and then they get tired that is when they sink and can drown.
If you want your dog to swim in the pool, the first thing that you want to do is put the leash on the dog. Bring the dog into the pool with you and let them swim around and then with the leash guide them to the steps and keep on doing that until they realize that is where they get out.
Repetition and consistency are so important to get the dog used to swimming in the water and swimming toward the stairs to get out. My suggestion is to always be outside with your dog when they are getting acclimated to swimming in the water.
Just remember that a dog’s psychology and a child’s psychology are very alike. They will try to push their limits as much as they can.