This study explores the application of double-walled carbon nanotubes (DWCNTs) in underwater acoustic materials, aiming to overcome the low-frequency absorption limitations of traditional materials. Four main aspects were investigated: first, the mechanical properties of armchair-type DWCNTs were calculated based on molecular dynamics, and the equivalent mechanical parameters of the DWCNTs-reinforced rubber composites were computed by the Halpin-Tsai model. Secondly, a 6 × 6 transfer matrix model based on orthotropic anisotropic materials was established to predict sound absorption properties, validated by COMSOL Multiphysics simulations. Again, the influence laws of six micro-macro key parameters of the reinforcing materials on the sound absorption characteristics were explored. Finally, a comprehensive multi-gradient and multi-parameter optimization study of the underwater acoustic functional material was carried out by the Bayesian Optimization and Hyperband (BOHB) optimization algorithm. The absorption bandwidth (α ≥ 0.65) of the optimized underwater acoustic functional composites spans from 0.299 kHz to 20 kHz, indicating its broadband absorption capability for practical applications. These findings advance the development of enhanced underwater acoustic materials.