During winter, molluscivorous sea ducks often form high density feeding flocks in association with patchily distributed prey, although few studies have documented the substrate and prey characteristics where sea ducks (tribe: Mergini) aggregate and thus what constitutes high-value sea duck foraging habitat. Sea duck surveys were conducted and benthic grab samples collected at sites with different benthic substrate characteristics to compare macroinvertebrate community assemblages in relation to densities of three species of sea ducks: (Surf (Melanitta peispicillata), Black (M. americana), and White-winged (M. fusca) scoters (hereafter “oscoters” when combined) during winter (2010–2011) in Narragansett Bay, Rhode Island, an urbanized estuary in the Northwest Atlantic. Overall, the highest densities of scoters (104 ± 17 per 0.5 km²) were found over sand substrates with homogeneous assemblages of infaunal prey. Significantly lower densities of scoters (21 ± 4 per 0.5 km²) were associated with mixed sand-gravel-mud substrates that supported epifaunal assemblages and patchily distributed infauna. Mean energy densities (kcal/g) were higher among infaunal (0.64–4.49) vs. epifaunal (0.17–0.53) prey. Overall biomass (g/m²) of polychaetes was higher in mixed substrates, and biomass of infaunal bivalves did not significantly differ among habitat type. However, infaunal prey may have been less accessible to scoters at sites with mixed substrates due to a barrier effect created by high-densities of epifauna with low energetic value. Thus, sand-substrate sites supporting infaunal benthic communities may provide high-quality feeding habitat for scoters wintering in the Northwest Atlantic. Understanding the influences of benthic habitat characteristics and macroinvertebrate prey assemblages on distribution of feeding scoters is particularly important for managing sea ducks in areas with increasing anthropogenic development in the coastal zone.