  We present a theoretical study on the electronic, dynamical, and
photocatalytic properties of single-layer ReS$_2$ under uniaxial and shear
strains. The single-layer ReS$_2$ shows strong anisotropic responses to
straining. It remains dynamically stable for a wide range of $x$-axial strain,
but becomes unstable for 2\% $y$-axial compressive strain. The single-layer
ReS$_2$ is calculated to be an indirect bandgap semiconductor, and there is an
indirect$-$direct bandgap transition under 1$-$5\% $x$-axial tensile straining.
The single-layer ReS$_2$ is predicted incapable of catalyzing the water
oxidation reaction. However, 1$-$5\% $y$-axial tensile strain can enable the
single-layer ReS$_2$ for overall photocatalytic water splitting. Besides, the
single-layer ReS$_2$ can also catalyze the overall water splitting and be most
efficient under acidic water solutions with pH=3.8.
