  We present a Very Long Baseline Interferometry image of the water maser
emission in the nuclear region of NGC3393. The maser emission has a linear
distribution oriented at a position angle of $\sim -34\degr$, perpendicular to
both the kpc-scale radio jet and the axis of the narrow line region. The
position-velocity diagram displays a red-blue asymmetry about the systemic
velocity and the estimated dynamical center, and is thus consistent with
rotation. Assuming Keplerian rotation in an edge-on disk, we obtain an enclosed
mass of $(3.1\pm 0.2) \times 10^7 M_{\sun}$ within $0.36\pm 0.02$ pc ($1.48\pm
0.06$ mas), which corresponds to a mean mass density of $\sim10^{8.2} M_{\sun}$
pc$^{-3}$. We also report the measurement with the Green Bank Telescope of a
velocity drift, a manifestation of centripetal acceleration within the disk, of
$5\pm 1$ km s $^{-1}$ yr$^{-1}$ in the $\sim3880$ km s$^{-1}$ maser feature,
which is most likely located along the line of sight to the dynamical center of
the system. From the acceleration of this feature, we estimate a disk radius of
$0.17\pm 0.02$ pc, which is smaller than the inner disk radius ($0.36\pm 0.02$
pc) of emission that occurs along the midline (i.e., the line of nodes). The
emission along the line of sight to the dynamical center evidently occurs much
closer to the center than the emission from the disk midline, contrary to the
situation in the archetypal maser systems NGC4258 and NGC1068. The outer radius
of the disk as traced by the masers along the midline is about 1.5 pc.
