  We use new precision measurements of black hole masses from water megamaser
disks to investigate scaling relations between macroscopic galaxy properties
and supermassive black hole (BH) mass. The megamaser-derived BH masses span
10^6-10^8 M_sun, while all the galaxy properties that we examine (including
stellar mass, central mass density, central velocity dispersion) lie within a
narrow range. Thus, no galaxy property correlates tightly with M_BH in ~L*
spiral galaxies. Of them all, stellar velocity dispersion provides the tightest
relation, but at fixed sigma* the mean megamaser M_BH are offset by -0.6+/-0.1
dex relative to early-type galaxies. Spiral galaxies with non-maser dynamical
BH masses do not show this offset. At low mass, we do not yet know the full
distribution of BH mass at fixed galaxy property; the non-maser dynamical
measurements may miss the low-mass end of the BH distribution due to inability
to resolve the spheres of influence and/or megamasers may preferentially occur
in lower-mass BHs.
