Water-resistant and environmentally friendly sodium-alginate-based films have been investigated to develop functional materials to extend the food’s shelf-life. A water-stable alginate-based film was prepared, employing both the internal and external gelation approach in the presence of CaCl2. To apply this film to food packaging and thus preserve food quality, the aim of this work is to perform a chemical and physical characterization of the proposed materials, evidencing the main features and stability under different work conditions. Water contact angle measurements showed a value of 65°, suggesting an important reduced hydrophilic character of the obtained alginate films due to the novel CaCl2-induced compacted polymer network. The film’s stability was thus checked through swelling measurements in water after varying pH, temperature, and ionic strength. The film was stable at high temperatures and not pH-responsive. Only highly concentrated salt-based solutions negatively affected the proposed packaging, causing a large swelling. Furthermore, a water-based polyphenolic extract from grape (Vitis vinifera L.) pomace waste was embedded inside the films in different amounts in order to confer additional properties. The extract’s polyphenolic content (evaluated from HPLC/MS-MS measurements) endowed the films’ UV-light screening and enhanced antioxidant properties. These important findings suggest the additional potential role of these films in protecting food from light deterioration. The stability of these hybrid films was also checked by observation, as the polyphenols’ presence did not largely alter the alginate network that occurred yet was water-resistant under the described work conditions.