We use new precision measurements of black hole (BH) masses from water
megamaser disks to investigate scaling relations between macroscopic
galaxy properties and supermassive BH mass. The megamaser-derived BH
masses span 10⁶-10⁸ {M}_⊙ ,
while all the galaxy properties that we examine (including total stellar
mass, central mass density, and central velocity dispersion) lie within
a narrower range. Thus, no galaxy property correlates tightly with
{M}_{BH} in ˜L* spiral galaxies as traced by megamaser
disks. Of them all, stellar velocity dispersion provides the tightest
relation, but at fixed {σ }_* 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 appear to 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 an inability to resolve their spheres of influence and/or
megamasers may preferentially occur in lower-mass BHs.