  The T2K off-axis near detector, ND280, is used to make the first differential
cross section measurements of muon neutrino charged current single positive
pion production on a water target at energies ${\sim}0.8$ GeV. The differential
measurements are presented as a function of muon and pion kinematics, in the
restricted phase-space defined by $p_{\pi^+}>200$MeV/c, $p_{\mu^-}>200$MeV/c,
$\cos \theta_{\pi^+}>0.3$ and $\cos \theta_{\mu^-}>0.3$. The total flux
integrated $\nu_\mu$ charged current single positive pion production cross
section on water in the restricted phase-space is measured to be
$\langle\sigma\rangle_\phi=4.25\pm0.48 (\mathrm{stat})\pm1.56
(\mathrm{syst})\times10^{-40} \mathrm{cm}^{2}/\mathrm{nucleon}$. The total
cross section is consistent with the NEUT prediction ($5.03\times10^{-40}
\mathrm{cm}^{2}/\mathrm{nucleon}$) and 2$\sigma$ lower than the GENIE
prediction ($7.68\times10^{-40} \mathrm{cm}^{2}/\mathrm{nucleon}$). The
differential cross sections are in good agreement with the NEUT generator. The
GENIE simulation reproduces well the shapes of the distributions, but
over-estimates the overall cross section normalization.
