  We present a wide-field optical imaging search for electromagnetic
counterparts to the likely neutron star - black hole (NS-BH) merger
GW190814/S190814bv. This compact binary merger was detected through
gravitational waves by the LIGO/Virgo interferometers, with masses suggestive
of a NS-BH merger. We imaged the LIGO/Virgo localization region using the
MegaCam instrument on the Canada-France-Hawaii Telescope. We describe our
hybrid observing strategy of both tiling and galaxy-targeted observations, as
well as our image differencing and transient detection pipeline. Our observing
campaign produced some of the deepest multi-band images of the region between
1.7 and 8.7 days post-merger, reaching a 5sigma depth of g > 22.8 (AB mag) at
1.7 days and i > 23.1 and i > 23.9 at 3.7 and 8.7 days, respectively. These
observations cover a mean total integrated probability of 67.0% of the
localization region. We find no compelling candidate transient counterparts to
this merger in our images, which suggests that either the lighter object was
tidally disrupted inside of the BH's innermost stable circular orbit, the
transient lies outside of the observed sky footprint, or the lighter object is
a low-mass BH. We use 5sigma source detection upper limits from our images in
the NS-BH interpretation of this merger to constrain the mass of the kilonova
ejecta to be Mej < 0.015Msun for a 'blue' (kappa = 0.5 cm^2 g^-1) kilonova, and
Mej < 0.04Msun for a 'red' (kappa = 5-10 cm^2 g^-1) kilonova. Our observations
emphasize the key role of large-aperture telescopes and wide-field imagers such
as CFHT MegaCam in enabling deep searches for electromagnetic counterparts to
gravitational wave events.
