I'm not sure a dog can swim for too long.
I beg to differ, a dog if properly conditioned
can swim for quite a long time, just as long as a human.
I swim my dogs from mid /end of May until end of September in the Delaware river against the current. The dogs have no leash or collar on; all I take is a tug to keep them motivated and focused. I go myself in the water so they have a distance target and they swim against the current of the river. At the beginning of the season I start out with about 15 min , by the end of the season they last as long as 75 minutes. I do it 2-3 times a week. Most important is after the swim excercise, a canine correct meal, to replenish the used up energy. IMO only the raw food diet can supply this with lots of raw meat and meaty bones. .....after a swimming day the dogs sleep really well.