  We present H$_2$O production rates for comet C/2012 S1 (ISON) derived from
observations of [OI] and OH emission during its inbound leg, covering a
heliocentric distance range of 1.8-0.44 AU. Our production rates are in
agreement with previous measurements using a variety of instruments and
techniques and with data from the various observatories greatly differing in
their projected fields of view. The consistent results across all data suggest
the absence of an extended source of H$_2$O production, for example sublimation
of icy grains in the coma, or a source with spatial extent confined to the
dimensions of the smallest projected field of view (in this case $<$ 1,000 km).
We find that ISON had an active area of around 10 km$^2$ for heliocentric
distances R$_h$ > 1.2 AU, which then decreased to about half this value from
R$_h$=1.2-0.9 AU. This was followed by a rapid increase in active area at about
R$_h$=0.6 AU, corresponding to the first of three major outbursts ISON
experienced inside of 1 AU. The combination of a detected outburst in the light
curve and rapid increase in active area likely indicates a major nucleus
fragmentation event. The 5-10 km$^2$ active area observed outside of R$_h$=0.6
AU is consistent with a 50-100% active fraction for the nucleus, larger than
typically observed for cometary nuclei. Although the absolute value of the
active area is somewhat dependent on the thermal model employed, the changes in
active area observed are consistent among models. The conclusion of a 50-100+%
active fraction is robust for realistic thermal models of the nucleus. However
the possibility of a contribution of a spatially unresolved distribution of icy
grains cannot be discounted.
