The environmental similarity scores between two locations are essential in ballast water risk assessment (BWRA) models used to estimate the potential for non-indigenous species introduction and guide management strategies aiming to minimize biodiversity loss and economic impacts. Previous BWRA models incorporate annual-scale environmental data, which may overlook seasonal variability. In this study, the differences in monthly sea surface temperature and salinity data were calculated at global ports and incorporated in a BWRA model. The environmental similarity scores were then calculated between the ballast water source and destination locations for ships arriving at Canadian coastal ports using monthly and annual-scale models for statistical comparison. As salinity and temperature vary seasonally in specific regions, there were significant differences in calculated environmental similarity risk values using annual and monthly-scale models. The results suggest that BWRA based on annual-scale data might underestimate environmental similarity scores. In contrast, methods incorporating monthly data can provide a more sensitive assessment to inform ballast water management practices.