The article discusses the hidden costs of multi-use at sea, focusing on transaction costs that arise from legal, insurance, health and safety, and governance challenges. As ocean space is increasingly used for production purposes such as food, energy, and resource extraction, competition for space becomes a concern. Multi-use at sea involves combining different industries and technologies in the same area, which can lead to both benefits and challenges. The article analyzes five case studies across Europe to understand the combined transaction costs of multi-use and suggests ways to reduce them. Multi-use at sea remains relatively new in the European context. Until recently, individual commercial operators were hesitant to engage in multi-use due to perceived risks and costs. The EU-funded UNITED project aims to demonstrate the benefits and challenges of multi-use by examining five pilot projects that combine economic activities such as renewable energy production, aquaculture, and tourism. The project highlights the obstacles in the path of arriving at profitable multi-use, including legal aspects, insurance, risk, and uncertainty, which determine whether multi-use can be implemented successfully. The article discusses the concept of transaction costs, which include the costs of negotiating, enforcing contracts, and managing risks. These costs are influenced by factors such as legal frameworks, insurance, and governance. The successful implementation of multi-use depends not only on the costs and benefits for individual activities but also on societal costs and benefits. Transaction costs also influence investment decisions, as they include planning, decision-making, permits, and dispute resolution. The analysis of the five pilots shows that transaction costs are significant and vary depending on the governance model and the type of activities involved. The article identifies several key risks, including the difficulty of obtaining full insurance coverage, adverse weather conditions, water quality issues, and the challenge of decommissioning facilities. These risks can lead to increased costs, delays, and reduced project viability. The article also discusses the importance of governance in multi-use projects, highlighting the need for clear regulations, coordination between stakeholders, and the development of a supportive legal framework. The governance models observed in the pilots range from strictly planned, top-down approaches to more flexible, adaptive models. The article suggests that a hybrid model could be beneficial, combining the strengths of different governance approaches. Insurance is another critical factor in multi-use projects, as the costs of insurance can be high due to the nature of the activities involved. The article discusses the challenges of obtaining insurance for multi-use projects, including the impact of market conditions and the need for clear contractual agreements to mitigate risks. Overall, the article emphasizes the importance of addressing transaction costs and governance challenges to facilitate the successful implementation of multi-use at sea. It highlights the need for collaboration between stakeholders, the development of a supportive legal framework, and the reduction of transaction costs to make multi-use more attractive and viable.The article discusses the hidden costs of multi-use at sea, focusing on transaction costs that arise from legal, insurance, health and safety, and governance challenges. As ocean space is increasingly used for production purposes such as food, energy, and resource extraction, competition for space becomes a concern. Multi-use at sea involves combining different industries and technologies in the same area, which can lead to both benefits and challenges. The article analyzes five case studies across Europe to understand the combined transaction costs of multi-use and suggests ways to reduce them. Multi-use at sea remains relatively new in the European context. Until recently, individual commercial operators were hesitant to engage in multi-use due to perceived risks and costs. The EU-funded UNITED project aims to demonstrate the benefits and challenges of multi-use by examining five pilot projects that combine economic activities such as renewable energy production, aquaculture, and tourism. The project highlights the obstacles in the path of arriving at profitable multi-use, including legal aspects, insurance, risk, and uncertainty, which determine whether multi-use can be implemented successfully. The article discusses the concept of transaction costs, which include the costs of negotiating, enforcing contracts, and managing risks. These costs are influenced by factors such as legal frameworks, insurance, and governance. The successful implementation of multi-use depends not only on the costs and benefits for individual activities but also on societal costs and benefits. Transaction costs also influence investment decisions, as they include planning, decision-making, permits, and dispute resolution. The analysis of the five pilots shows that transaction costs are significant and vary depending on the governance model and the type of activities involved. The article identifies several key risks, including the difficulty of obtaining full insurance coverage, adverse weather conditions, water quality issues, and the challenge of decommissioning facilities. These risks can lead to increased costs, delays, and reduced project viability. The article also discusses the importance of governance in multi-use projects, highlighting the need for clear regulations, coordination between stakeholders, and the development of a supportive legal framework. The governance models observed in the pilots range from strictly planned, top-down approaches to more flexible, adaptive models. The article suggests that a hybrid model could be beneficial, combining the strengths of different governance approaches. Insurance is another critical factor in multi-use projects, as the costs of insurance can be high due to the nature of the activities involved. The article discusses the challenges of obtaining insurance for multi-use projects, including the impact of market conditions and the need for clear contractual agreements to mitigate risks. Overall, the article emphasizes the importance of addressing transaction costs and governance challenges to facilitate the successful implementation of multi-use at sea. It highlights the need for collaboration between stakeholders, the development of a supportive legal framework, and the reduction of transaction costs to make multi-use more attractive and viable.