Marine-derived nutraceuticals have arisen as promising sources of biologically active compounds with potential applications in preventive nutrition and health promotion. Marine organisms such as fish, crustaceans, mollusks, macroalgae and microalgae provide a range of bioactive peptides, polysaccharides, lipids and polyphenols with antioxidant, anti-inflammatory, antimicrobial, immunomodulatory, cardiometabolic and neuroprotective activities. This review summarizes the major classes of marine-derived nutraceuticals, their molecular and cellular mechanisms, gastrointestinal fate, bioaccessibility and bioavailability, and their relevance to human health. The biological pathways through which marine peptides, polysaccharides, omega-3 lipids, phlorotannins, carotenoids and related compounds may modulate oxidative stress, inflammatory signaling, lipid metabolism, intestinal function and immune regulation are given special emphasis. The review also deals with the influence of source, extraction, processing, chemical structure, food matrix and digestion on nutraceutical activity and bioavailability. Emerging delivery approaches including encapsulation, nanoemulsions, liposomal systems and other advanced formulations for improving stability and biological availability are discussed. In vitro, animal, and human studies are critically reviewed to distinguish mechanistic potential from clinical proven benefits. Also discussed are safety issues such as contaminants, allergenicity, compositional variability and regulatory issues. Finally, the opportunities related to sustainable use of marine biomass, valorization of by-products, precision nutrition and advanced biotechnology are highlighted. Increased standardization of bioactive characterization, dosage, preparation, safety assessment and clinical endpoints will be essential for the transition of marine-derived nutraceuticals from experimental investigation to reliable, evidence-based health applications.