To reveal the roles of pores and fissures in the rock-weakening effect of water, this study used porosity to quantitatively characterize pores and fissures in rocks. Based on the results from mechanical tests on rock specimens with different porosities and degrees of saturation, the combined effects of the porosity and water content on the mechanical properties and deformation behaviour of rocks were investigated. The test results indicated that the rock-weakening effect of water was markedly more significant if the pores and fissures in the rock were highly developed. The test data showed that the ratio of saturated specimen strength/dry specimen strength of porous rock can be even lower than 0.1. The least-squares regression method was used to derive the impact of rock porosity on rock weakening due to the effect of water. Based on the results, a practical engineering method for modifying the mechanical parameters of wet rocks with highly developed pores and fissures was proposed. This method can be used to calculate the stress–strain state of rocks more accurately.