But their rescue of the amphibian man so that he cannot be dissected by the ruthless overseer of the laboratory turns the movie into a warm story about kindness and bravery.

For del Toro, The Shape of Water is "a love letter to classic cinema" that he has dreamed of directing for years.

When the Guadalajara-born filmmaker first watched the 1954 horror film Creature from the Black Lagoon, he felt that an underwater sequence involving an amphibious humanoid and a female swimmer could have been romantic. He thought they would end up together, but it didn't happen in that film.

He says he wanted it to end well for the creature, as well as make a film that "heals people the way they need to be healed".

Haunted by the idea for more than two decades, del Toro even quit directing the $150-million sci-fi film Pacific Rim Uprising early, and took up The Shape of Water, which was being made for only $19.3 million.

"It's not an easy movie to make for that amount of money. We needed a lot of time to do it carefully," says del Toro, adding that the project was both ambitious and beautiful.