My first lab use to go canoe with me, he would swim half the way on a ten mile float.
Several years we shot a goose that swam out, my lab went after it. The goose got thru some stumb fields, so we couldn't get the boat to help. We watch him thru binoclars, he had to have went 500 yards before running the goose down.
I have heard that retrievers use less energy swiming that running. But cold water is another story.