Normalized Difference Water Index.
For MODIS (bands at 857 and 1241).
NDWI = (rho857-rho1241)/(rho857+rho1241)
It seems that for SPOT, the SWIR band could be used instead of the 1240. The 860 band is the NIR.
Reference: Gao, B. C., 1996. NDWI - A normalized difference water index for remote sensing of vegetation liquid water from space. Remote Sensing of Environment 58, 257-266.