Marra Sea Country stretches across the Limmen Bight region in the southern Gulf of Carpentaria, Northern Territory (NT), and covers multiple jurisdictions: Aboriginal land, the Territory’s Limmen Bight Marine Park, and the Commonwealth’s Limmen Marine Park. The cultural knowledge in this region is rich and well-documented, however, the marine biodiversity values in the region have not been studied in detail. The li-Anthawirriyarra land and sea rangers, Marra Traditional Owners, scientists from James Cook University and Charles Darwin University co-designed a marine benthic survey using a mixed methods approach. We found extensive and diverse seagrass - a meadow more than 75 km in length with dugong feeding trails throughout, occurrence of a rare sponge, fields of feather stars and coral bleaching. The knowledges underpinning the cultural and biodiversity values will be woven together to form a monitoring framework and then a program for future monitoring by the rangers.