The SNO+ detector operated initially as a water Cherenkov detector. The implementation of a sealed cover gas system midway through water data taking resulted in a significant reduction in the activity of Rn222 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 B8 solar neutrino analysis with a total livetime of 282.4 days and an analysis threshold of 3.5 MeV. The B8 solar neutrino flux is found to be (2.32-0.17+0.18(stat)-0.05+0.07(syst))×106 cm-2 s-1 assuming no neutrino oscillations, or (5.36-0.39+0.41(stat)-0.16+0.17(syst))×106 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.