Introduction Proximity and access to water have long been central to human culture and accordingly deliver countless societal benefits. Over 200 million people live on Europe's coastline, and aquatic environments are the top recreational destination in the region. In terms of public health, interactions with ‘blue space’ (eg, coasts, rivers, lakes) are often considered solely in terms of risk (eg, drowning, microbial pollution). Exposure to blue space can, however, promote health and well-being and prevent disease, although underlying mechanisms are poorly understood. Aims and methods The BlueHealth project aims to understand the relationships between exposure to blue space and health and well-being, to map and quantify the public health impacts of changes to both natural blue spaces and associated urban infrastructure in Europe, and to provide evidence-based information to policymakers on how to maximise health benefits associated with interventions in and around aquatic environments. To achieve these aims, an evidence base will be created through systematic reviews, analyses of secondary data sets and analyses of new data collected through a bespoke international survey and a wide range of community-level interventions. We will also explore how to deliver the benefits associated with blue spaces to those without direct access through the use of virtual reality. Scenarios will be developed that allow the evaluation of health impacts in plausible future societal contexts and changing environments. BlueHealth will develop key inputs into policymaking and land/water-use planning towards more salutogenic and sustainable uses of blue space, particularly in urban areas. Ethics and dissemination Throughout the BlueHealth project, ethics review and approval are obtained for all relevant aspects of the study by the local ethics committees prior to any work being initiated and an ethics expert has been appointed to the project advisory board. So far, ethical approval has been obtained for the BlueHealth International Survey and for community-level interventions taking place in Spain, Italy and the UK. Engagement of stakeholders, including the public, involves citizens in many aspects of the project. Results of all individual studies within the BlueHealth project will be published with open access. After full anonymisation and application of any measures necessary to prevent disclosure, data generated in the project will be deposited into open data repositories of the partner institutions, in line with a formal data management plan. Other knowledge and tools developed in the project will be made available via the project website (www.bluehealth2020.eu). Project results will ultimately provide key inputs to planning and policy relating to blue space, further stimulating the integration of environmental and health considerations into decision-making, such that blue infrastructure is developed across Europe with both public health and the environment in mind.

Evidence suggests that living near blue spaces such as the coast, lakes and rivers may be good for health and wellbeing. Although greater levels of physical activity (PA) may be a potential mechanism, we know little about the types of PA that might account for this. To explore the mediating role of: a) 'watersports' (e.g. sailing/canoeing); b) 'on-land outdoor PA' in natural/mixed settings (e.g. walking/running/cycling); and, c) 'indoor/other PA' (e.g. gym/squash) in the relationships between residential blue space availability and health outcomes. Using data from the Health Survey for England (n = 21,097), we constructed a path model to explore whether weekly volumes of each PA type mediate any of the relationships between residential blue space availability (coastal proximity and presence of freshwater) and self-reported general and mental health, controlling for green space density and a range of socio-economic factors at the individual- and area-level. Supporting predictions, living nearer the coast was associated with better self-reported general and mental health and this was partially mediated by on-land outdoor PA (primarily walking). Watersports were more common among those living within 5kms of the coast, but did not mediate associations between coastal proximity and health. Presence of freshwater in the neighbourhood was associated with better mental health, but this effect was not mediated by PA. Although nearby blue spaces offer potentially easier access to watersports, relatively few individuals in England engage in them and thus they do not account for positive population health associations. Rather, the benefits to health from coastal living seem, at least in part, due to participation in land-based outdoor activities (especially walking). Further research is needed to explore the mechanisms behind the relationship between freshwater presence and mental health.