Water has substantial effects on the tensile failure behavior of rock engineering materials in excavations and underground openings. However, our understanding of the tensile failure process of wet rock and its water-weakening mechanism is limited. In this sense, a series of uniaxial tension tests, coupled with acoustic emission (AE) signal monitoring, were carried out on dry, natural, and saturated marble rock. Importantly, the dominant frequencies of AE waveforms recorded during the tension loading were analyzed by a newly introduced quantitative statistical method. The test results show that the increasing water content of marble rock leads to significant strength reductions while having no relation with the twin-peak feature of dominant frequency bands of AE waveforms. The statistical analysis suggests that the dominant micro-tensile failures always appear before micro-shear failures and increase slightly with an increase in water content. The weak pore water pressure causes the initial release moments of micro-tensile failures for natural and saturated rocks to be advanced. With increasing water contents, more micro-tensile failures carrying less energy occur, while the amount and energy of micro-shear failure both reduce significantly. The mechanism involved in the drastic tensile strength reduction is likely to be friction weakening because of the water films, which is manifested by a significant loss in the amount and energy of micro-shear failures. This work provides a reasonable explanation for the tensile strength reduction of water-bearing rock by connecting the rock micro-failure and the dominant frequency of AE waveform signals released.