Description: These data include salinity and oxygen isotope measurements of water samples collected from coastal sites along the Gulf of Maine between 2003 and 2015. In particular, a suite of samples were collected along the coast of Maine, east of Penobscot Bay, on a monthly basis between April 2014 and March 2015. These data also include several freshwater samples collected from the Kennebec and Penobscot Rivers on a semi-monthly basis in 2014 and 2015. For the water samples with sample IDs starting with DSW, JSW, NSW, or OSW: The water samples were collected by hand from shore or boat using French square glass bottles with phenolic polycone lined caps. Salinity was measured using a Oakton SALT 6+ handheld salinity meter. Oxygen isotopes were measured using a Picarro L2130-i Isotopic Liquid Water Analyzer with an attached autosampler. Water samples with sample IDs starting with ASW were collected from shore. Samples with sample IDs starting with DMC 2010 were collected at the flowing seawater laboratory at the Darling Marine Center. Samples with sample IDs starting with Summer 2011 were collected from a boat. For these last 3 sample types (ASW, DMC 2010, Summer 2011): Salinity was measured with YSI Professional Plus salinity meter and oxygen isotopes were measured using a Picarro L1102-i Isotopic Liquid Water Analyzer with an attached autosampler. Data from Owen et al., 2008 and Wanamaker et al. (2006, 2007) was collected from the flowing seawater laboratory at the Darling marine center. Salinity was measured using a YSI model 85 oxygen, conductivity, salinity, and temperature system and oxygen isotopes were measured using a dual-inlet VG/Micromass SIRA (CO2–H2O equilibration method at 30 °C for 12 h).