  We present first results of the H_2O Southern Galactic Plane Survey (HOPS),
using the Mopra radiotelescope with a broad band backend and a beam size of
about 2'. We have observed 100 square degrees of the southern Galactic plane at
12mm (19.5 to 27.5GHz), including spectral line emission from water masers,
multiple metastable transitions of ammonia, cyanoacetylene, methanol and radio
recombination lines. In this paper, we report on the characteristics of the
survey and water maser emission. We find 540 water masers, of which 334 are new
detections. The strongest maser is 3933Jy and the weakest is 0.7Jy, with 62
masers over 100Jy. In 14 maser sites, the spread in velocity of the water maser
emission exceeds 100km/s. In one region, the water maser velocities are
separated by 351.3km/s. The rms noise levels are typically between 1-2Jy, with
95% of the survey under 2Jy. We estimate completeness limits of 98% at around
8.4Jy and 50% at around 5.5Jy. We estimate that there are between 800 and 1500
water masers in the Galaxy that are detectable in a survey with similar
completeness limits to HOPS. We report possible masers in NH_3 (11,9) and (8,6)
emission towards G19.61-0.23 and in the NH_3 (3,3) line towards G23.33-0.30.
