Actor Sam Worthington and actress Zoe Saldana reprise their respective roles as Jake Sully and Neytiri, and the movie also casts new faces, with Titanic actress Kate Winslet playing the pregnant wife of the Metkayina clan's chief. Winslet had special training for the role, enabling her to hold breath for seven minutes and 15 seconds underwater.

Accounting for a big part of the film, the underwater scenes are possibly the most eye-catching and technically challenging. A variety of fantastic creatures are featured, ranging from "skimwing", a large flying fish-like creature, to "ilu", an easily tamed creature which looks like plesiosaurs.

For director Cameron, who recently attended an online interview with Chinese journalists alongside producer Jon Landau, the sequel is his love letter to the ocean.

"I love being underwater. I love the oceans. I love these incredible animals. I love the beauty in the coral reefs or in a kelp forest, that sort of thing … I'm just putting together my two great loves in life: film — telling stories with images — and the ocean," says the director.

One of Cameron's most sensational news was his 2012 record-setting solo dive to get down 10,908 meters, reaching the bottom of Mariana Trench in the Pacific Ocean, the world's deepest frontier.

Describing himself as an explorer who has spent thousands of hours underwater, Cameron reveals that he has made eight deep ocean diving expeditions around the world, and made a lot of documentaries about the sea, exemplified by the Emmy Award-winning Secrets of the Whales, a TV nature documentary that has followed a variety of whales in 24 locations over three years.