In recent years, surface-enhanced Raman spectroscopy (SERS) has developed rapidly and is used for the detection of molecules and biomolecules in liquids. However, few studies have focused on SERS using a water surface as the substrate. A floating metal film on water is desirable for an enhanced SERS performance. In this work, silver nanoparticles (Ag NPs) encased in poly(vinylpyrrolidone) films (Ag-PVP films) were synthesized on the surface of an aqueous solution by room temperature electron reduction. A floating silver film on a water surface was thereby achieved and is reported for the first time. The synthesized Ag-PVP film is an excellent flexible substrate for SERS and has other potential applications. Using the floating silver film as a flexible SERS substrate, 10–11 M of 4-aminothiophenol, 10–6 M of riboflavin, 10–9 M of 4-mercaptobenzoic acid, 10–7 M of 4-mercaptophenol, and 10–7 M of 4-aminobenzoic acid are identified, demonstrating potential use for the floating substrate in the liquid-phase detection of molecules.