Strengthening the participation of local communities is decisive for projects involving natural resources management in coastal areas. Where local communities actively participate in the management of coastal resources and development projects, environmental impacts have significantly lessened. However, communities are not properly involved in development projects. In this paper, we analyze the coastal development project to build a variant canal to Cartagena Bay on the Caribbean coast of Colombia, South America, and the participation of local communities in natural resources management. Using Vicente Conesa’s method to assess environmental impacts and designing a compliance tool, we found communities are not included in development projects and the management of natural resources. Yet, local communities are autonomously enacting spaces of participation to protect coastal and marine ecosystems that have been proven to be effective in preventing environmental impacts. Using this case study, we propose policy instructions to contribute to the conservation of marine and coastal ecosystems and the rights to food security, information, participation, and environmental justice of local communities in South America.