Truly, if you’re not following the official Avatar social channels, you really should. There’s a regular stream of behind-the-scenes photos that will alternately excite and befuddle you. (The latter is especially true given how much of the movies are realized during an extensive post-production process.) Today’s latest quizzical post features Oscar-winning actress Kate Winslet doing, er, something underwater. In the sequel Winslet co-stars as Ronal, a free diver of the Metkayina, the reef tribe of Na’vi that are, apparently, central to the new story, which makes the photo, featuring Winslet at …