There are several theories about how the Portuguese Water Dog came to be, but despite all the theoretical reasoning, the Portuguese Water Dog was definitely a large part of the Portuguese sea faring trade for generations. The dog was a working dog, and a hard working one at that. He was used for all types of dangerous water-related jobs on fishing boats, and was only replaced by mechanical technologies in the last 150 years.
Whether it was the Visigoths who brought the descendants of today's Portuguese Water Dog to the shores of Portugal in BC 400, or the Moors who brought them in the 8th century AD, from the time they were introduced to Portugal, the Portuguese Water Dog - also known as the Portuguese Fishing Dog or the Lion Dog - took over the hearts and minds of the men in the fishing industry.
Interestingly enough, it is not thought that the ancestors of the Portuguese Water Dog were actually raised to be a water dog. In fact, the commonly recognized ancestors that were begun near the Chinese-Russian board around BC 700 were used as herders. They worked with the livestock raised there like sheep, goats, and cattle.
This explains why the dogs were initially used by the Portuguese fishermen to "herd" fish into nets. The dogs would literally swim after the fish near the nets and "chase" them towards the nets. After a while, the retriever side of the dogs came out and they were used to retrieve things like broken fishing equipment or objects accidentally dropped into the ocean. The dogs were such excellent swimmers, whether it was in the warm seas of the Portuguese coast or the icy waters of Iceland, they were trained to be messengers. The dogs would carry messages to other boats in the vicinity, or they would be ship to shore messengers.
The dogs worked on both larger fishing vessels and on small boats. This brought about the need for there to be a wide variety of sizes of Portuguese Water Dogs. Thus today we have males that can range from 20 to 23 inches and 42 to 60 pounds, and females that range from 17 to 21 inches and 35 to 50 pounds. The smaller dogs were used on the smaller personal fishing boats, while the larger boats kept the larger dogs.
Of course all this water work led the dog to develop special physical attributes to help them survive in the frigid waters. The dogs have slightly webbed paws, and this helps them swim. Portuguese Water Dogs also have water-proof coats that do not shed; this would help the dog stay warm in the water. The dog was even groomed specially to help him swim: the "lion cut", which shaves the dog's hind quarters, muzzle, and the base of the tail helped the dogs move their legs in the water, while the body was kept warm by the rest of the coat.