In coastal zone management (CZM), scientific knowledge can help enrich and underpin the development of policy options by providing insight into ecosystems and their management, the use of ecosystem goods and services, and ecological limits to the exploitation of natural resources. Due to the large array of interests and stakeholders involved in CZM, however, it is often complicated to produce and use knowledge which is perceived to be credible, legitimate and salient. The scholarly literature advocates employing collaborative and participatory approaches, such as the development and use of boundary objects, to enhance the production and use of knowledge in CZM with the aim of enriching decision-making processes. This paper empirically explores two assessment systems as boundary objects in order to address the question ‘To what extent and in what way do boundary objects contribute to enriched coastal zone management?’. Our analysis suggests that for a boundary object to contribute to enriched CZM, the need for it to be credible is less important than the need for all stakeholders involved to perceive it and its development process as being legitimate to their interests. Secondly, without a direct ‘policy window’ the boundary object has little chance of directly enhancing decision-makers’ knowledge.