  Dark matter that participates in baryon-number violating interactions can
annihilate with baryons if the dark matter particle is not protected under
discrete symmetries. In this paper we investigate the dark matter - baryon
annihilation in color-triplet extensions of the Standard Model, in which a
fermionic dark matter can be kinematically stable within a small mass range
near the proton mass. We demonstrate that the DM's annihilation with nucleons
can be probed to stringent limits at large-volume water Cherenkov detectors
like the Super-Kamionkonde experiment, with the mediator scale $m_\Phi$
constrained up to $10^{7}$ GeV. In case of a Majorana light dark matter, this
constraint is weaker yet close in magnitude to that from neutron-antineutron
oscillation. In the Dirac DM case, the dark matter- nucleon annihilation gives
much stronger bounds than that from the uncertainties of the neutron decay
lifetime. In a limited range of the DM mass above $m_p+m_e$, the DM-nucleon
annihilation bound can be higher than the requirement from the DM's stability
in the Universe. Given the strong limits from Super-Kamionkonde, we find it
below the current experimental capabilities to indirectly detecting the dark
matter- nucleon annihilation signal in diffuse Galactic gamma rays and neutron
star heating.
