What kinds of knowledges are produced through material engagement with and within underwater environments? How is productivity performed underwater by communities of local divers, marine biologists, design researchers and marine species? The bay of Valsaline in the city of Pula is a biodiverse underwater environment hosting on its shores one of the oldest recreational dive clubs in Croatia, the “CPA Pula”. The knowledge of the local divers goes back to 1965 and can help us trace the long-term relations between human and marine life in the bay. Their accounts reflect anger over political disengagement in underwater urban planning and related declining biodiversity. The disappearance of marine species has been exacerbated by recent construction of a canalisation structure in the middle of the bay. Our project “Interfacing the Ocean” (2023-2027, Swiss National Science Foundation) aims to engage with these issues through design research, together with marine biology and anthropology. Discussing field research activities such as joint dives, interviews, and autoethnography, as well as regenerative design proposals, I will reflect on productivity and time with/in the sea and its costal communities.