5000 Miles with our Shetland Sheepdog
My wife and I lived aboard our 40 footer for two years and never had a problem with this issue. "Guy" loved to go places with us, but never fell overboard. We did keep a harness on him with a handle on the top and a boat hook close at hand just in case, but he was never a problem. We took him all over the Bahamas (he peed on every deserted island there), and though he was not a large dog, he did weigh about thirty pounds. Your Retriever should be able to swim readily, and you should be able to get him into the dingy, but I would guess that you already know that.