r.water.outlet needs the outlet coordinates in the same CRS as the input file.
Since your "computational region is North:1570225.7, East:739499.1, South:1319713.3, west:474246" and in your screen shot says "EPSG: 32643 (OTF)" means your are mixing units. The outlet point should be meters as CRS and not in lon/lat.
My screen is a bit different from your despite I am also runing QGIS 2.18.14 and it allows me to pick up the point from the screen. Try to use the Processing Toolbox is on the right of my picture.