Mine have all been natural swimmers, but I have had to coax some into the water the first time. Cassidy loved to swim, but she was really bad at it, lol! She always looked on the verge of drowning, with her head held way high and her front legs slapping the water.
Dena would wade out up to her belly as a puppy, but she wouldn't go further for awhile. I think she was about 8 months old the first time she actually swam - we'd toss the ball a little bit further each time until finally she had to swim a few strokes to get it, and then there was no stopping her. She was a great swimmer, she'd glide through the water like a fish, with nary a splash.
Keefer followed Dena and was swimming the first time he was in water deep enough to swim in. He was swimming a few months younger than she was her first time. Halo was around deep water at least once and maybe twice before she would do more than splash around in the shallows, but once she did it the first time, she was fearless. She might have been even younger than Keefer was, and she's the only dog we've had that jumped off a dock into a swimming pool at a dock diving event the first time she'd ever even seen a dock or a pool!
Personally, I'd never just toss a dog in, I like starting off a beach, preferably without waves. Keefer and Halo will both swim in the ocean now, but they learned in the bay, where currents are gentle.