Titanium (IV) oxide is a reference photocatalytic material for a number of chemical processes of interest in the energy transition, including the reduction of water into hydrogen gas. Density functional theory calculations have previously revealed that the decomposition into hydrogen and hydroxyl ions could be catalyzed by a subsurface oxygen vacancy (VO). However, these theoretical investigations have been carried out with models considering only isolated water molecules. We investigate the role of the subsurface VO at different levels of coverage up to full monolayer (ML) coverage. We observe that migration of the vacancy concomitantly to water decomposition is mandatory to obtain an exothermic pathway. Furthermore, the migration of the VO only occurs in the presence of an under-coordinated surface oxygen atom. As coverage and decomposition increase, surface oxygen atoms are saturated which inhibit further migration of the subsurface VO; hence the VO shows no catalytic effect on the splitting of water into hydrogen and hydroxyl ions at full water coverage.