National audienceIn the general context of development of marine activities, many protections and rules are set up in the coastal sea. Yet, paradoxically, the knowledge of marine activities, especially recreational activities often remains fragmented. This chapter propose a state of the art of observation of human activities at sea methods, and discusses its methodological, scientific and social issues, from a perspective of knowledge and management of the coastal sea.Dans un contexte général de développement des activités maritimes, la mer côtière voit se mettre en place de nombreux dispositifs de protection et de réglementation. Pourtant, de manière assez paradoxale, la connaissance des activités maritimes et en particulier des activités ...