I don't have any dogs (wife is allergic) but my mum has two big collie mountain rescue dogs.
They wanted to take them canoeing on some of the lochs near their house but weren't sure about how safe it would be. They knew the dogs could swim but weren't sure what would happen if they should capsize.
So...they tested it, close to shore, capsized the boat dogs and all! Apparently the results were interesting, instead of swimming one of the dogs panicked and tried to climb on top of my mum for buoyancy! Bearing in mind it's a big dog even with lifejackets there were some issues staying above the water for all involved!
Final analysis was, not taking the dogs on the canoe anymore.