I know it's probably far fetched, but when I saw the trailer and the two characters fighting a Krait with bubbles all around, the first thing which came to my mind was World of Warcraft. There was this one expansion where we explored huge underwater zones. In order to make fighting less tedious, the devs chose to enable characters to "dock on" the ground, so that battles could be done almost like on land. Running had a different animation, with wider strides.
Now we venture to Cantha and the home of the Deep Sea Dragon - the horror of the depths. Maybe this is a kind of underwater expansion, with the difference that - through magic or something else - players can dock-on the sea ground end explore it in a new movement style.