I'm working on a new character for the Beach scene and when I load the scene it says  "the following is missing from /D:/backups/sysdat/barbarian/projects/redoc/locations/beach/backdrops/sky-water-hd-wallpapers.jpg". I know there's a folder with some wallpapers, but which one is the right one? I've tried adding each of them as a backdrop in daz 3d and they aren't matching up with the scene.  What am I missing?

Those are the right backdrops.  You'll have to pick the right one depending on the camera up/down angle.  Just use whichever one lines up with its water-line slightly below the water line from the scene.  If none of them are quite right, just take the closest one in photoshop and slide it up or down a bit. 

Those were made for the 16x9 shots though.  For the vertical ones you'll need to either crop the backdrop ahead of time, or render the whole thing in 16x9 and then crop the final render.