Modern Newfoundlands are slooooow swimmers. Why? They are big dogs with lots of bone, lots of coat, and substance.
Compare their swimming to this flat-coat, which is a breed with some feathering but no coarse bone:
Now, both of those dogs have a common ancestor. I highly suspect that if the flat-coat were taken to Newfoundland in the eighteenth century, it would be put to work immediately on the fishing fleet. If the Newfoundlands in the youtube video were taken to Newfoundland in eighteenth century, I think the fishermen would leave them on the dock!
I thought it might be interesting to train a modern Newfoundland dog as a retriever, but I’m sure it could never be trial quality.