The geometry, electronic, and photocatalytic properties of vdW heterostructures of GeC and Janus transition metal dichalcogenides MSSe (M = Mo, W) monolayers are investigated by performing first-principles calculations. Two different possible models of GeC-MSSe heterostructures are presented with an alternative order of chalcogen atoms at opposite surfaces in MSSe. The most favorable stacking pattern of both models is dynamically and energetically feasible. A direct type-II band alignment is obtained in both models of understudy heterobilayer systems. In particular, a greater Rashba spin polarization is demonstrated in model 1 (GeC-WSSe) than model 2 (GeC-MoSSe) caused by the alternative order of chalcogen atoms and larger SOC effect of heavier W than Mo atoms. More interestingly, the appropriate band alignments of model 1 with the standard water redox potentials enable its capability to dissociate water into H+/H2 and O2/H2O. The GeC-MSSe are proposed promising for in future electronic, spintronics, and photocatalytic water splitting.