There is fragmented understanding of the interactions of environmental and socioeconomic systems, sectors and processes involved in coastal water quality evolution around the world. Environmental policies applying to the coastal environment also have either a land or a sea perspective. As such, the policies may be ill-adapted to support improvements of complex coastal environments that represent the interface of and link the land and sea systems. For instance, environmental regulations in the Baltic Sea region have not yet managed to sufficiently mitigate coastal eutrophication. To make policies relevant and effective, stakeholder perceptions of the environmental and socioeconomic interactions that determine the state of the coastal environment also need to be included in modelling for policy support. Such modelling approaches can facilitate knowledge integration and be a basis for synergistic efforts for coastal environment improvement. For such facilitation and for bridging the gap between land and sea policy perspectives on coastal environmental management, this study considers and models stakeholder-perceived environmental and socioeconomic interactions in a linked land-coast-sea system. To determine stakeholder perceptions of these interactions, multiple actors representing various inland, coastal, and marine sectors in the Swedish water management district Northern Baltic Proper have been engaged in a series of workshops to codevelop a system network diagram (SND) for the district’s land-coast-sea system. The co-developed SND is used in semi-quantitative modelling of coastal water quality behaviour under various human pressure and hydro-climatic conditions. Results show that synergistic multi-scale, transboundary and multi-actor measures are needed to improve coastal water quality, even locally. The measures also need to address the impacts of both currently active sources and legacy sources (accumulated over time from past to-present inputs) of nutrients. A fully-quantitative systems dynamics (SD) model is further developed, based on the system behaviour insights gained from the semi-quantitative interaction modelling. This is used for scenario analysis that can support policy by determining effective nutrient management pathways for meeting coastal water quality challenges under various possible socioeconomic developments along with climatic change. Nutrient contributions to coastal waters from different sectors in the study region are then quantified considering the water and nutrient exchanges among the sectors and the natural water environments throughout the linked land-coast-sea system. Scenario outcomes are analysed based on a set of key performance indicators for the coastal environment. Such results can guide the development of effective environmental policies for coastal water quality and ecosystem management in the studied Baltic system and other coastal areas around the world. The booklet of abstracts can be found here: https://knowledge4policy.ec.europa.eu/event/2021-eu-conference-modelling-policy-support-collaborating-across-disciplines-tackle-key_en