To address the dispersion problem of fluidized solidified soils used as novel scour protection materials for offshore wind turbine foundations during underwater construction, this study investigates the underwater performance of high-water-content dredged sludge solidified with a slag–fly ash–cement composite binder. The effects of the thickening agent hydroxypropyl methylcellulose (HPMC) and the water-reducing agent polycarboxylate ether (PCE) on underwater constructability were systematically examined. By adjusting the dosages of HPMC and PCE, flowability and anti-dispersion tests were conducted to explore their synergistic interaction with the slag–fly ash–cement solidification system. Fourier Transform Infrared Spectroscopy (FTIR) was employed to analyze the functional group compositions of HPMC and PCE and their mechanisms of influence on material properties. The results indicate that when the HPMC dosage is 0.4%, the anti-dispersion performance of the fluidized solidified soil is significantly improved, and the corresponding flowability reaches a peakl. On this basis, the addition of 0.15% PCE allows the material to meet the requirements for pumpable underwater construction. Considering both overall performance and economic feasibility, the PCE dosage should not exceed 0.2%. Overall, optimizing the HPMC/PCE dosages enables a favorable synergistic response of the composite system and markedly improves the underwater construction performance of flowable solidified soil.