To tie a water knot in webbing, first tie an overhand skeleton knot and dress the knot so the webbing lays flat (see step #1 in this illustration). Then take the end of a second piece of webbing and follow the first piece of webbing in reverse (similar to tying a figure 8 bend or a figure 8 follow through). Dress the knot leaving 3+ inch tails.
Note that the tails on a webbing water knot "creep" a tiny amount each time they are loaded and unloaded (somewhere around 4/100ths of an inch). Over time (okay, over a long time), the tails can shrink and eventually get pulled through the knot. Because of this, it's especially important to inspect the tails of previously-tied webbing each time you use it.
When an overhand bend is tied in round rope (i.e., cord), it is often called a ring bend.
vRigger costs less than a rope.