Underwater adhesion has long been limited by interfacial water's paradoxical role as both bonding mediator and failure initiator. We present a confined hydration adhesive tape (CHAT) that harnesses water as a molecular architect through spatial hydration management. By confining water penetration to sub-8-micrometer depths, we create a dynamic interface where hydration-activated hydrogen bonds enable adaptive, high-density interfacial connections, and hydrophobic nanodomains maintain bulk integrity via entropic water exclusion. This orchestrated hydration yields an interfacial toughness of 6 kilojoules per square meter (>1.8x literature benchmarks; 1.4 to 3.8x commercial tapes), while preserving stability across harsh conditions (pH 1 and 13, 3.5% saline). Multiscale experiments and simulations reveal water's triple role as a hydrogen bond catalyst at the interface, a dynamical reorganizer of supramolecular networks, and a mechanical decoupler of interfacial adhesion/bulk cohesion. By establishing interfacial water as a design variable rather than a compromise, CHAT opens avenues for marine, biomedical, and industrial applications where water-resistant adhesion is critical.