Perlite-Metal Syntactic Foam (P-MSF) is a novel lightweight material with good specific strength and excellent energy absorption capabilities. To analyse its suitability in marine applications, P-MSF has been immersed for 2 years in natural flowing seawater as well as freshwater. The change of mass and mechanical properties has been studied as a function of exposure time. Results indicate a slow degradation of mechanical properties that can be attributed to a change of the macroscopic deformation mechanism. Interestingly, no evidence of significant corrosion was observed. Instead, the change in mechanical properties is triggered by the sedimentation of oxides and sulphates within the expanded perlite particles. Implications towards the long-term viability of such P-MSF in marine applications are discussed.