A shallow, stable boundary layer is ubiquitous over the cool waters of the Gulf of Maine in summer. This layer affects pollutant transport throughout the region by isolating overlying flow from the surface. In this paper, we explore how the stable boundary layer is formed and describe its characteristics. The temperature profile of the lowest 1--2 km of the atmosphere over the Gulf of Maine is remarkably similar regardless of transport time over water or the time of day when the flow left the land, provided only that the flow is offshore. This similarity is forced by the (roughly) constant water temperature and the (roughly) constant temperature of the free troposphere over the continent. However, the processes leading to the similar profiles are quite different depending on the time of day when the flow crosses the coast. Air leaving the coast at night already has a stable profile, whereas air leaving the coast at midday or afternoon has a deep mixed layer. In the latter case, the stable layer formation over the water is of interest. Using observations of surface fluxes, profiles, and winds on the NOAA Research Vessel Ronald H. Brown from the 2004 International Consortium for Atmospheric Research in Transport and Transformation (ICARTT)/New England Air Quality Study, we show that the formation of the stable layer, which involves cooling a roughly 50- to 100-m-deep layer by 5--15 K, occurs within 10 km and a half hour after leaving the coast. The internal boundary layer near shore is deeper than predicted by standard relationships. Historical data are explored and also show deeper internal boundary layers than predicted. We also describe one exceptional case where a 200-m-deep neutral layer was observed and discuss the degree of isolation of the stable boundary layer and its duration.