The surface vibrational spectrum of water at biological interfaces is often interpreted as having 'ice-like' and 'liquid-like' components. Here we show that the vibrational spectrum of water at both water-lipid and water-protein interfaces greatly simplifies upon H/D isotopic dilution, which is inconsistent with the presence of 'ice-like' structures. The changes in the spectra as a function of isotope content can be explained by intramolecular coupling between bend and stretch vibrations of the water molecules.