Oceanographic data and their related simulations have a key role in addressing EU and UN societal challenges in marine environments. Visualising marine data is challenging for different visual-communication intents and audiences, despite existing guidelines on the subject. A main visual-design limitation for existing techniques is the co-visualisation of multiple hydrodynamic field attributes in an accessible, comprehensible and engaging manner. This paper addresses this limitation in two ways: firstly, existing techniques for cartographic-oriented design of waterlines are adopted and extended towards multivariate hydrodynamic field datasets. Secondly, experimental results on the intermixing of different visual-channel mappings for hydrodynamic attribute data are presented in a case study on ocean-flow patterns around the Hebrides island chain (United Kingdom). The results demonstrate a comprehensible simultaneous co-visualisation of up to five unique, independent scalar attributes within a single-figure layout while preserving the geographic context. Moreover, insights and best-practices are stated in conclusion of the experimental case study to help oceanographic practitioners adopt the presented technology in their professional workflows.