Deriving a catchment boundary in GRASS

[N R A]

Can anyone tell me if it's possible in GRASS to simply derive a catchment 
boundary. All we want to do is to point to a point on a river and for the 
system to derive the catchment to that point.

I've tried the 'r.watershed' function, but it doesn't seem to work with 
the spearfish test data - although I think it's more likely that I'm not 
using it correctly.

I'm thinking of creating a new function, which will mean the users will 
only have to point at a spot on the screen, in a similar way to 'd.where' 
and the system will go on to 
i) derive the catchment boundary and
ii) calculate various stats (catchment area, percentage of various raster 
maps, e.g. soils, potential evaporation, etc., in the derived catchment 
area)

Anyone done this and/or could point me at the code ?

Any help much appreciated
Thanks,

Steve C.