User blog comment:Commdor/Mass Effect 3: Leviathan trailer released/@comment-1679409-20120824222359/@comment-1221773-20120825014901

Well, it's only an idea, but I wouldn't count Yamm out because the description fails to explicitly note jellyfish as fauna (my impression from the description is that arthropods and algae are predominant, but there are other species too), the weather is fierce (other water worlds like Trident and Proteus are equally stormy but were colonized anyway), or Shepard wasn't masked (the trailer footage may be taken from a game where helmets/masks were toggled off, or perhaps Yamm's poisonous algae doesn't pose a breathing hazard).

Although if I had to bet, I'd say that Leviathan's water world is a new planet in a new system that the DLC adds.