Abstract: To address the issues of weak interfacial bonding, poor toughness, and slow curing in underwater epoxy adhesives, this study prepared a cardanol-phenol modified epoxy adhesive via the Mannich reaction, systematically investigating the effect of the bisphenol ratio on the curing agent performance and the underwater bonding behavior of the adhesive. The results indicate that when the molar ratio of cardanol to phenol is 1∶1, the curing agent exhibits optimal comprehensive performance. The resulting adhesive achieves an underwater bond shear strength of 11.85 MPa, representing a 184.2% improvement compared to the unmodified system, and the failure mode transitions from interfacial failure to cohesive failure, demonstrating effective synergistic enhancement. Curing kinetics analysis shows that the modified system has lower curing peak temperatures and a lower apparent activation energy of 45.22 kJ/mol, both of which are lower than those of the single cardanol-modified system. This indicates that the phenolic structure effectively enhances the curing reaction activity. This study provides an effective strategy for developing high-performance underwater epoxy adhesives.