This chapter explores the narration of risk in marine produce from Iwaki in Fukushima Prefecture, Japan. The release of radioactive matter into the Pacific Ocean following the 2011 earthquake, tsunami and nuclear disaster in north-east Japan is the largest-ever accidental marine pollution incident. In Fukushima, this has affected coastal communities for whom fishing is economically, socially and culturally significant. We explore the interplay between two different narratives around radiation risk in marine produce. One concerns dispelling ‘harmful rumours’ around safety of marine produce, claiming that as marine radiation can be known and controlled, raising concerns over safety is harmful to the recovery of affected locales. The second is an alternative narrative emerging from local-level actors (municipal government, fisheries cooperatives, NGOs) focused on the inherent uncertainty associated with complex coastal ecosystems. Transparency and participation in data collection act to develop collective understandings of an ‘acceptable’ level of uncertainty. Drawing on field research from Iwaki, we illustrate that while these two narratives are distinct, they are by no means polar opposites. Common to both are wider questions about trust in the people and institutions producing knowledge about water and about what can be known with certainty in water environments.