  The Neil Gehrels Swift Observatory carried out prompt searches for
gravitational wave (GW) events detected by the LIGO/Virgo Collaboration (LVC)
during the second observing run ("O2"). Swift performed extensive tiling of
eight LVC triggers, two of which had very low false-alarm rates (GW 170814 and
the epochal GW 170817), indicating a high confidence of being astrophysical in
origin; the latter was the first GW event to have an electromagnetic
counterpart detected. In this paper we describe the follow-up performed during
O2 and the results of our searches. No GW electromagnetic counterparts were
detected; this result is expected, as GW 170817 remained the only astrophysical
event containing at least one neutron star after LVC's later retraction of some
events. A number of X-ray sources were detected, with the majority of
identified sources being active galactic nuclei. We discuss the detection rate
of transient X-ray sources and their implications in the O2 tiling searches.
Finally, we describe the lessons learned during O2, and how these are being
used to improve the \swift\ follow-up of GW events. In particular, we simulate
a population of GRB afterglows to evaluate our source ranking system's ability
to differentiate them from unrelated and uncatalogued X-ray sources. We find
that $\approx$60-70% of afterglows whose jets are oriented towards Earth will
be given high rank (i.e., "interesting" designation) by the completion of our
second follow-up phase (assuming their location in the sky was observed), but
that this fraction can be increased to nearly 100% by performing a third
follow-up observation of sources exhibiting fading behavior.
