Discovery of Candidate H$_2$O Disk Masers in AGN and Estimations of
  Centripetal Accelerations
  Based on spectroscopic signatures, about one-third of known H$_2$O maser
sources in active galactic nuclei (AGN) are believed to arise in highly
inclined accretion disks around central engines. These "disk maser candidates"
are of interest primarily because angular structure and rotation curves can be
resolved with interferometers, enabling dynamical study. We identify five new
disk maser candidates in studies with the Green Bank Telescope, bringing the
total number published to 30. We discovered two (NGC1320, NGC17) in a survey of
40 inclined active galaxies (v_{sys}< 20000 kms^{-1}). The remaining three disk
maser candidates were identified in monitoring of known sources: NGC449,
NGC2979, NGC3735. We also confirm a previously marginal case in UGC4203. For
the disk maser candidates reported here, inferred rotation speeds are 130-500
kms^{-1}. Monitoring of three more rapidly rotating candidate disks (CG211,
NGC6264, VV340A) has enabled measurement of likely orbital centripetal
acceleration, and estimation of central masses (2-7x10^7 M_\odot) and mean disk
radii (0.2-0.4pc). Accelerations may ultimately permit estimation of distances
when combined with interferometer data. This is notable because the three AGN
are relatively distant (10000<v_{sys}<15000 kms^{-1}). As signposts of highly
inclined geometries at galactocentric radii of \sim0.1-1pc, disk masers also
provide robust orientation references that allow analysis of (mis)alignment
between AGN and surrounding galactic stellar disks, even without
interferometric mapping. We find no preference among published disk maser
candidates to lie in high-inclination galaxies, providing independent support
for conclusions that central engines and galactic plane orientations are not
correlated. (ABRIDGED)
