Little is known of the types that went into its genetic makeup, although it was probably a random-bred mix of old English, Irish and Portuguese working dogs.

The St. John's water dog was also an ancestor to the large and gentle Newfoundland, probably through breeding with Rafeiro do Alentejos brought to the island by the generations of Portuguese fishermen who had been fishing offshore since the 15th century.

The classic tuxedo markings of the St. John's water dog commonly manifest in Labrador Retriever mixes.

The dogs were described as having a short thick coat, rudder-like tail, high endurance, and a great love of swimming.

It was web-footed, could swim extremely fast, dive with great ease, and bring up any thing from the bottom of the water.

They are not attended with a driver, nor any person to guide them; but after having delivered their loading, they return immediately to the woods, where they are accustomed to be fed with dried fish, &c.” "We have seen first-class dogs smooth-coated, wavy-coated, and curly.

This dog is known by his smooth, though slightly wavy and glossy coat, being the foundation of the wavy-coated retriever already alluded to (page 89).

Their main overseas destination, the UK, imposed a rigorous long-term quarantine on all imported animals, especially dogs (1885) as part of the eradication of rabies.

In the 1970s, Canadian author Farley Mowat had tried to save them by crossing his St. John's water dog, named "Albert", with a Labrador Retriever.