Some scenes (underwater and action) are in 48 fps. Not the whole movie.

... Excerpts from James Cameron Using a ‘Simple Hack’ to Achieve High Frame Rate on ‘Avatar: The Way of Water’ :

James Cameron will be using a “simple hack” to bring high-frame rate projection to standard cinemas for his upcoming “Avatar: The Way of Water.”

Previous attempts to distribute high frame rate versions of films globally have been largely unsuccessful with theatre owners and audiences, due to high projector upgrade costs and a perceived lack of cinema aesthetics.

Instead, Cameron will be working within established digital cinema specifications which allow for digital cinema packages (DCP) screening at 48fps, which a majority of 3D enabled cinemas can project.

“We’re using [high frame rate] to improve the 3D where we want a heightened sense of presence, such as underwater or in some of the flying scenes. For shots of just people standing around talking, [high frame rate] works against us because it creates a kind of a hyper realism in scenes that are more mundane, more normal. And sometimes we need that cinematic feeling of 24fps,” said Cameron.

“Can theatres support variable frame rate, switching back and forth within the movie between 24fps and 48fps? The answer is no, they just run it at 48fps. In any part of the scene that we want at 24fps, we just double the frames. And so, they actually show the same frame twice, but, but the viewer doesn’t see it that way. And so, we just we’re essentially using a simple hack to use the high frame rate platform that already exists.”