The underwater curing agents have demonstrated promising potential in various applications, especially in underwater repair engineering, yet have proven considerable challenging. Here, we report a cardanol-based curing agent for epoxy resin that achieves robust adhesion to steel and concrete substrates both in air and underwater. Cardanol, paraformaldehyde, and m-phenyldimethylamine are selected as the polymeric monomers to synthesize curing agent by Mannich reaction in the absence of chemical cross-linker agents. The coating is completely cured within 46 ± 1 min in air and 54 ± 2 min under water with an adhesion of 0 or 1 and a hardness of 5H. The impact strength, shear strength, and tensile strength of coating on underwater concrete were 9.58 ± 0.41 kJ·m−2, 13.1 ± 0.3, and 10.5 ± 0.2 MPa, respectively, demonstrating exceptional flexibility and mechanical strength as well as favorable hydrophobicity. This work paves the way for the rehabilitation of underwater drainage network for urban infrastructure and water conservancy projects.