There was somebody here on the board whose dog swam out once and kept on going. It may have been at a lake, I don't remember, but it took awhile to get the dog back, and in the meantime they were freaked OUT!!!! I think Lynn_P may have been there, and it might have been G-Burg's dog who went for a swim - for some reason I'm thinking it was a Wildhaus pup, but I could be mistaken.
We don't have access to a pool, so my dogs learned to swim in the SF Bay. Because it's shallow for a long way out, and currents aren't really a factor, other than the very slow (over a period of several hours) tide changes, it's pretty safe. In a pinch I could easily wade out after them because even though it's deep enough that they have to swim it's not deep enough that I'd have to.
They like to play in the water near the edge and will only swim out if we throw a ball for them to retrieve. The only time I was ever concerned was one time with Keefer and his Jolly Ball. He picks it up off the ground by biting at it, which works just fine on dry land, but not so much in the water. He'd bite at it and it would duck underwater, and get pushed a little further out. He tried and tried and tried, but fortunately recognized that he was getting tired, and came back to shore.
This was the first time we brought the JB to the park and I was ready to give up and donate it to the Bay, but after panting a few minutes he decided he wasn't going to leave it behind and he swam back out. Again, it kept ducking underwater and getting pushed further out, he got tired, and came back. More panting, and then third time's a charm - he swam out again and finally got it by the rope, but now it's half full of water and weighs about 10 pounds! He towed that darned thing all the way back, silly boy, but by that time I was afraid he was going to drown himself trying!!!
Here he is, trying to get that ball
Bringing it back
They also swim in the ocean, but we were going to Fort Funston for over a year before they actually swam there for the first time. We'd just throw the ball along the beach, parallel to the water, and they'd splash around in the shallows. It wasn't until we saw other people throwing toys out for their dogs to swim for that we decided it would be okay to try it. We're careful NOT to heave the ball out as far as we can, just a short distance, and only at low tide when the waves are small. I'd never do it in larger waves and rough surf, and since unless we throw a ball they don't swim, we don't worry even if we're there and the conditions aren't conducive to swimming. But you can't count on that with every dog. We also know that if they lose the ball in the surf they won't kill themselves trying to find it. If they're out there looking around, we can call them back and they'll give up on the ball and come to shore.
They are very strong swimmers, much better than I am, but we try to be prudent about where and how they swim. Since I know Keefer is nuts for his Jolly Ball, we don't bring that to the beach. If I have to wade in and drag him out of shallow water in the Bay, I can, but I couldn't do that if he gets in trouble in the ocean. You really have to know your own dog, and better safe than sorry.