Background: Crosslinker-free, pH-induced hydrogels offer a green alternative for food preservation but often lack mechanical robustness. Objective: In this study, we developed a ternary hydrogel from cod myofibrillar protein (CP), sodium alginate (SA), and Ulva prolifera-derived insoluble dietary fiber (IDF) to enhance structural and preservation properties. Methods: Hydrogels with 0–3% IDF were characterized to assess their texture, water-holding capacity (WHC), and microstructure. Based on the balance between reinforcement and macroscopic processability, the 2% IDF formulation was selected for the shrimp preservation trial, which was conducted over 15 days at 4 °C. Results: Incorporation of 2% IDF significantly increased gel hardness (from 278.0 ± 6.8 g to 393.0 ± 1.8 g; p < 0.01, n² = 0.87) and WHC (from 60% to 84.3 ± 2.1%; p < 0.01). In preservation tests, the CP-SA-IDF coating maintained TVB-N at 41.62 ± 3.7 mg/100 g, significantly lower than the control (78.65 ± 4.5 mg/100 g; p < 0.01) and inhibited microbial growth (TVC: 6.9 ± 0.3 log CFU/g vs. control 9.1 ± 0.4 log CFU/g; p < 0.05). A combined freshness index demonstrated superior overall preservation efficacy (0.82 vs. 0.49 in control; p < 0.05). Conclusions: IDF reinforces the CP-SA network via hydrogen bonding and physical entanglement, creating an effective edible coating for aquatic product preservation.