ABSTRACT Seawater has occupied an ambiguous place in anthropological categories of “nature” and “culture.” Seawater as nature appears as potentiality of form and uncontainable flux; it moves faster than culture—with culture frequently figured through land-based metaphors—even as culture seeks to channel water's (nature's) flow. Seawater as culture manifests as a medium of pleasure, sustenance, travel, disaster. I argue that, although seawater's qualities in early anthropology were portrayed impressionistically, today technical, scientific descriptions of water's form prevail. For example, processes of globalization—which may also be called “oceanization”—are often described as “currents,”“flows,” and “circulations.” Examining sea-set ethnography, maritime anthropologies, and contemporary social theory, I propose that seawater has operated as a “theory machine” for generating insights about human cultural organization. I develop this argument with ethnography from the Sargasso Sea and in the Sea Islands. I conclude with a critique of appeals to water's form in social theory.