There is increasing interest in the potential use of outdoor water environments, or blue space, in the promotion of human health and wellbeing. However, therapeutic nature-based practices are currently outpacing policy and the evidence base for health or wellbeing benefits of therapeutic interventions within blue space has not been systematically assessed. This systematic review aims to address the gap in understanding the impacts of blue space within existing interventions for targeted individuals. A systematic review was carried out, searching Google Scholar, SCOPUS, PubMed, etc. through to August 2017. Only blue space interventions were included that were specifically designed and structured with a therapeutic purpose for individuals with a defined need and did not include nature-based promotion projects or casual recreation in the outdoors. Thirty-three studies met the inclusion criteria and were assessed. Overall, the studies suggest that blue care can have direct benefit for health, especially mental health and psycho-social wellbeing. The majority of papers found a positive or weak association between blue care and health and wellbeing indicators. There was also some evidence for greater social connectedness during and after interventions, but results were inconsistent and mixed across studies with very few findings for physical health. This is the first systematic review of the literature on blue care. In summary, it has been shown that mental health, especially psycho-social wellbeing, can be improved with investment in blue spaces. Key areas for future research include improving understanding of the mechanisms through which blue care can improve public health promotion.