If you are worried about your dogs ability to swim, put a long line on it but don't heave it in, just encourage your dog to swim to you and lightly use the line to guide it in.
One of our favourite picnic spots is a few miles to the west of Aberdeen alongside the river Dee. There's easy access to the river which is wide and appears fairly slow moving at that point but we still use long training leads on our two before letting them venture anywhere near the water.
Having been fortunate enough to fish that particular stretch on a few occasions I'm well aware that the slow moving surface water hides a fast moving current just a few feet below which has caught out a few human swimmers over the years and could easily put an animal in danger.
Chaz, our worker, is a very strong swimmer and we have to keep a close eye on him as he does his "seal" impressions. Milly also likes water but only until it comes up to her underside at which point she wants back onto dry land, a good brisk towelling down and a gravy bone

Milly's also done an impression but, in her case it was of a black submarine

. She was running along Aberdeen beach and enjoying dashing through the pools of water that had collected around the groynes on the beach. Unfortunately one of the pools was much deeper than the rest and she completely disappeared for a second or so before surfacing.
After getting out of the water there was much head shaking and sneezing out quantities of the north sea before she warily approached the pool of water, sniffed at it and then gave it a good barking at before proceeding along the beach at a much more leisurely pace, giving the other groynes a very wide berth.
Bob.