This paper explores the determinants of foreign visitors’ participation in a conservation scheme for marine biodiversity. It also sheds light on respondents’ behaviour when confronted with collective and voluntary payment modes in the context of Contingent Valuation Method (CVM). The elicited conservation values concern two endangered species; the loggerhead turtle Caretta caretta and the monk seal Monachus monachus. The findings of this pilot study indicate that the main reasons for respondents’ participation are prior knowledge of species and attitude to management options. More importantly, they show evidence of sensitivity to the method of payment. Despite respondents state a lower Willingness-to-Pay (WTP) when a collective mechanism is used in place of a voluntary one, they are more likely to participate in the scheme with a higher certainty. Finally, the elicited WTP values of the two marine endangered species confirm that there is potential for internal funding which could sustain the operation of the National Marine Park of Zakynthos (NMPZ) in Greece.