From April through September of 2018, water-quality data were collected at 35 locations from select tributaries, embayments, and nearshore lake locations along New York's portion of Lake Ontario in support of the 2018 Cooperative Science and Monitoring Initiative, the Great Lakes Restoration Initiative, and the Lakewide Action Management Plan. These data include environmental, replicate, and blank samples of nutrients, suspended solids, sodium, and chloride, as well as field measurements of physical parameters (temperature, specific conductance, dissolved oxygen, pH, turbidity, phycocyanin relative fluorescence, chlorophyll relative fluorescence, photosynthetically active radiation, and phycocyanin:cholorphyll ratio).