Recent investigations have revealed that some transition-metal dichalcogenides (TMDs), such as MoS2 and WS2, are excellent candidates for high-efficiency photocatalysts for water splitting. However, the high recombination rate of photogenerated carriers greatly hinders their practical application. A promising solution involves developing novel TMDs-based van der Waals (vdW) heterostructures with type-II band alignment. In this study, we used first-principles calculations to design two new heterostructures - MoS2/BSe and WS2/BSe - as potential photocatalysts and investigated their structures, stabilities, and electronic and optical properties. We found that both MoS2/BSe and WS2/BSe vdW heterostructures are stable and possess inherent type-II band alignment, which significantly suppresses the recombination of photogenerated carriers. Furthermore, their band edges straddle the redox potential of water, making them suitable for use as photocatalysts in water splitting. They also possess significant built-in electric fields, relatively high carrier mobilities, and excellent abilities to absorb sunlight. Our theoretical findings should shed light on the design of novel TMD-based photocatalysts for water splitting and provide useful guidelines for future experiments.