  The SNO+ detector operated initially as a water Cherenkov detector. The
implementation of a sealed covergas system midway through water data taking
resulted in a significant reduction in the activity of $^{222}$Rn daughters in
the detector and allowed the lowest background to the solar electron scattering
signal above 5 MeV achieved to date. This paper reports an updated SNO+ water
phase $^8$B solar neutrino analysis with a total livetime of 282.4 days and an
analysis threshold of 3.5 MeV. The $^8$B solar neutrino flux is found to be
$\left(2.32^{+0.18}_{-0.17}\text{(stat.)}^{+0.07}_{-0.05}\text{(syst.)}\right)\times10^{6}$
cm$^{-2}$s$^{-1}$ assuming no neutrino oscillations, or
$\left(5.36^{+0.41}_{-0.39}\text{(stat.)}^{+0.17}_{-0.16}\text{(syst.)}
\right)\times10^{6}$ cm$^{-2}$s$^{-1}$ assuming standard neutrino oscillation
parameters, in good agreement with both previous measurements and Standard
Solar Model Calculations. The electron recoil spectrum is presented above 3.5
MeV.
