A water knot for making a loop is basically the same thing as tying an overhand on a bight. The only difference is, you are able to feed the free end around something (like a ring) first, and it's a bit easier to dress. You could just as easily pass the webbing through the loop first, and then tie an overhand on a bight by passing the ring through the knot (I hope that makes sense). If you want to do it like you're making a water knot, just tie one half of the water knot with a longer-than-usual tail, then feed the tail through the ring, and fold the tail back over itself and use it to tie the rest of the knot, just like you would if it were a different piece of webbing.