Tests show the water knot to reduce webbing strength around 30% (the CMC Rescue Field Guide App gives the water knot a 64% efficiency rating). This reduction in strength is a result of the bends in the knot and associated stress-concentration points. Bar tacks (sewn slings) are stronger because, instead of bends, stitching is used to weave and enforce the overlapped ends.  The overall breaking strength of bar tacks differ depending on manufacture and I have yet to find a study that directly compares the strength of bar tacks to water knots under similar circumstances – please send one if you got it!  Generally speaking neither type is prone to break under normal circumstances so no need to rush out and buy sewn slings to replace tied ones (although the lack of a bulky knot can be nice). Time is better spent understanding how the overall strength of webbing changes based on its configuration, composition, and use.