The Water Framework Directive (WFD) establishes that Programme of Measures (PoMs) to manage aquatic ecosystems have to be assessed to determine whether the benefits obtained from the good ecological status outweigh the costs. Stated preference methods have been widely applied to estimate non-market benefits of improved ecological status, assuming that respondents declare their true preferences that are stable over time. However, evidence on preference stability is mixed in the literature. The objective of this paper is to study preference stability towards water bodies improvement. As the WFD implementation has been delayed and have proven to be a greater challenge than expected, we focus on the effects of policy implementation failures on preference stability. We analyse two contingent valuation surveys to assess environmental values for the case of the Mar Menor (SE Spain) in years 2010 and 2017. We find higher protest response rate, changing indirect utility functions towards good ecological status and decreased WTPs in 2017. It indicates that public valuation might fail to adhere to rational economic premises when public authorities fail to reach environmental objectives and/ or PoMs are not correctly implemented. Decision making based on stated preferences should carefully consider potential biases emerging from management performance.