It is widely accepted that obligate aquatic mammals, specifically toothed whales, rely relatively little on olfaction. There is less agreement about the importance of smell among aquatic mammals with residual ties to land, such as pinnipeds and sea otters. Field observations of marine carnivorans stress their keen use of smell while on land or pack ice. Yet, one dimension of olfactory ecology is often overlooked: while underwater, aquatic carnivorans forage “noseblind,” diving with nares closed, removed from air-borne chemical cues. For this reason, we predicted marine carnivorans would have reduced olfactory anatomy relative to closely-related terrestrial carnivorans. Moreover, because species that dive deeper and longer forage farther removed from surface scent cues, we predicted further reductions in their olfactory anatomy. To test these hypotheses, we looked to the cribriform plate (CP), a perforated bone in the posterior nasal chamber of mammals that serves as the only passageway for olfactory nerves crossing from the periphery to the olfactory bulb and thus covaries in size with relative olfactory innervation. Using CT scans and digital quantification, we compared CP morphology across Arctoidea, a carnivoran clade at the interface of terrestrial and aquatic ecologies. We found that aquatic species from two lineages that independently reinvaded marine environments (Pinnipedia, Mustelidae), have significantly smaller relative CP than terrestrials. Furthermore, within aquatic lineages, diving depth and duration were strongly correlated with CP loss, with the extreme-diving elephant seals showing the greatest reductions. These observations suggest that CP reduction in carnivorans is an adaptive response to shifting selection pressures associated with re-invasions of marine environments, particularly foraging at great depths. Because the CP is fairly well preserved in fossils, using methods presented here to quantify CP morphology in extinct mammals could help define evolutionary patterns of olfactory loss across lineages that have independently committed to life in water.