Abstract: To address seepage defects in horizontal construction joints induced by layered placement of mass concrete in navigation hub projects, this paper conducts a comparative study on the impermeability performance of different interface treatment methods for horizontal construction joints. A total of seven groups of concrete specimens with different interface treatments for horizontal construction joints were prepared, and core samples were drilled from the actual construction site. Impermeability tests were carried out on horizontal construction joints under seven combined interface treatment conditions. The test results indicate that conventional mechanical chiseling and water jetting cause varying degrees of damage to the concrete at the joint surface, resulting in loose fine aggregates, surface cracking, and particle contamination. These defects readily form persistent seepage channels and fail to meet the impermeability requirements of hydraulic structures. The combined treatment of chiseling plus a 5 cm-thick mortar layer exhibits poor impermeability performance. The thin mortar layer cannot fully fill the irregular surface profile, loose defects, and microcracks generated by chiseling, and the mortar layer is easily damaged during subsequent concrete vibration. Direct application of a cement-based crystalline waterproof coating on the top surface of horizontal construction joints faces significant difficulties in surface cleaning, coating application, and field curing, leading to unstable construction quality and unsatisfactory impermeability performance. In comparison, applying the waterproof coating to the side surfaces of horizontal construction joints ensures more reliable construction quality and superior impermeability performance, and can serve as an effective auxiliary anti-seepage measure. The composite treatment method combining high-pressure water jetting or mechanical chiseling with a 20 cm-thick well-graded mortar-enriched concrete layer can effectively repair surface damage and irregularities. This layer acts as a buffer, alleviates interfacial restraint between the upper and lower concrete layers, and improves the overall bonding performance of the concrete. Featuring excellent impermeability and convenient construction, this optimized method has been applied to horizontal construction joints in mass concrete structures of the Pinglu Canal Hub Project and fully satisfies the project’s impermeability requirements. Nevertheless, the impermeability grade of treated construction joints still fails to reach the design standard of monolithic concrete. Construction joints remain relatively weak parts of concrete waterproofing systems. Strict quality control of layered concrete placement, vibration compaction, and interface treatment procedures is therefore essential throughout the construction process to ensure the overall impermeability performance of mass concrete structures.