I'll add my own take here, as we go all over New England and swim in all kinds of water.
Good water instinct is supposed to be part of the Golden temperament, but not all Goldens are bred carefully for it. Even if your dog has the genes for it, he still may be nervous at first. Don't force anything. Find a spot where the shore is gradual and the water is very calm so he has a chance to splash and play and get comfortable without having to actually swim.
As others have said, the first few times swimming can be really inelegant. Yes, they have an instinct for how to swim, but they're generally not efficient at first. If they don't make good forward progress, the body tends to tilt underwater, butt down, head up, and they end up splashing a lot. They may even panic a bit. That's why a gentle sloping shore is nice, so the dog gets more comfortable in the water but can still put his feet down if he starts to tilt.
The easiest way to motivate him is to play fetch with an older, confident dog. Your dog will want to chase out after the toy and the other dog, and will challenge himself to swim deeper.
Even so, it can take a few outings before your pup actually leaves his depth and tries to swim. And it'll take a couple more times after that before he really knows how to swim. He'll teach himself, though. Before he has fully taught himself, be prepared to help him if he needs it. You'll know he really grasps the concept once he stops splashing the surface with his front paws. His stroke will change and become more efficient, and that's when you know he can really swim.
Dogs don't need life jackets for ordinary swimming, but they absolutely do need them in any situation where a human might wear one, like on a boat trip. Most dog life jackets also have a strong handle between the shoulder blades, so if your dog does fall off the boat or the pier, you can haul him out much more safely than if you had to grab his collar.
You must have good recall if you want your dog to swim on open water. If your dog is birdy and has poor recall, it's completely possible for him to misjudge his ability and get into a dangerous situation. Goldens are usually strong swimmers, and they can be terribly poor decision makers when they're estimating how far they can swim. A Golden is perfectly capable of swimming farther and farther and farther after a duck until he drowns himself. And I've seen ducks who seemed to be trying to drown dogs. They let the dog get close, and then they flap ten or twenty feet away, and then they wait for the dog to catch up again, all the while drawing him further and further out. You MUST be able to call your dog off a bird if he's birdy.
No shoes. He needs his webbed feet in order to swim effectively. Try not to swim him from the same spots on the shore that fishermen use, and keep a pair of pliers and a wirecutter in your car if you're worried about fishhooks. You have to push the fishhook through, snip off the barbed part, and then pull it backwards and out. If you can't push it through because of its position, it's time for the emergency vet.
Here's the problems I've run into in the last decade or so:
Current. Dogs are generally wary of whitewater, but not always, and if the current is strong but doesn't disturb the surface, they're not good judges of its strength or its speed. We had a big scare when one of our favorite streams was running high and Jax got pulled downstream and fetch up against a log. He was pinned against it for a few seconds because he kept fighting against going under it (which would have washed him past it safely), so Andy and I both jumped into the brook to get him. He was fine, but it was a big scare.
Clam beds. We take the dogs to the local salt marsh all the time, and at certain tides, the clam beds are hidden under an inch or two of water. The dogs run across and slash up their paws. We haven't ever had a slash bad enough to require a vet visit, but it's obviously something to avoid.
Getting out is easier than getting in. The dogs will not plan ahead on how to get out of a body of water. So if it's a rocky shore with a two foot drop into the water, they'll happily jump in, swim around, and then they're really sad when they can't get out. This applies to docks and pools, too. You may have to show your dog how to get out at least the first time and maybe the first few times. After that, he should pick it up.
Don't fear swimming! It's written right into their genes, and for some dogs, not much makes them happier than a swim. When we run or hike, the dogs will splash into every pond, lake, or stream along the way, just for kicks. It's the best low impact exercise possible, and it's a joy to watch a Golden Retriever crashing into the water.
