Prince Edward Island (PEI) is the only Canadian province 100% dependent on groundwater for both irrigation and drinking water. Groundwater resources on PEI, which also sustain surface water ecosystems, have faced compounding stresses in recent years due to water demands and nitrate contamination from the extensive agricultural industry. There is also increasing concern regarding the impacts of droughts and saltwater intrusion due to climate change. Freshened Offshore Aquifers (FOAs) may represent a new freshwater resource for PEI, and for similar island settings worldwide. SOURCE is an exploratory study of freshened submarine groundwater resources offshore of PEI in the Gulf of St. Lawrence. Here, the growth and collapse of the Laurentide Ice Sheet led to the formation of extensive networks of paleo-channels and lakes, superimposed on fractured continental sandstones. A 9-week research expedition onboard the RV Maria S Merian was conducted in the fall of 2021 to collect geophysical data, conduct sediment coring, and undertake bathymetric mapping. Data from this expedition are being interpreted in a numerical model (HydroGeoSphere) of 3D variable-density groundwater flow and salt transport to assess the evolution of freshened groundwater in the Gulf of St. Lawrence and to investigate the hydrogeologic controls. Concurrent onshore (coastal) hydrogeological investigations are being conducted on PEI to elucidate onshore-offshore aquifer connections and regional aquifer dynamics. A developed island-wide groundwater elevation map and numerical groundwater model reveal how regional hydrogeological systems influence the island groundwater dynamics, water budget, and connections to offshore aquifers. Coastal drone-based thermal mapping and sampling has been conducted at low tide to provide insight into submarine groundwater discharge patterns and mechanisms and associated nutrient loading. Understanding onshore and offshore groundwater dynamics and their connections will help ensure that the implementation of recent groundwater legislation for this vulnerable island province is grounded in hydrogeologic science.