We report here that a dense liquid formed by spontaneous condensation{,} also known as simple coacervation{,} of a single mussel foot protein-3S-mimicking peptide exhibits properties critical for underwater adhesion. A structurally homogeneous coacervate is deposited on underwater surfaces as micrometer-thick layers{,} and{,} after compression{,} displays orders of magnitude higher underwater adhesion at 2 N m-1 than that reported from thin films of the most adhesive mussel-foot-derived peptides or their synthetic mimics. The increase in adhesion efficiency does not require nor rely on post-deposition curing or chemical processing{,} but rather represents an intrinsic physical property of the single-component coacervate. Its wet adhesive and rheological properties correlate with significant dehydration{,} tight peptide packing and restriction in peptide mobility. We suggest that such dense coacervate liquids represent an essential adaptation for the initial priming stages of mussel adhesive deposition{,} and provide a hitherto untapped design principle for synthetic underwater adhesives.