The use of seawater (typically in the form of saline nasal sprays or rinses) to support or prevent colds has some scientific basis, though the evidence is moderate. Saline solutions, which mimic the salt concentration of seawater, are commonly recommended for nasal irrigation. Clinical studies indicate that saline nasal irrigation can help reduce symptoms of upper respiratory tract infections, including the common cold, by clearing nasal passages, thinning mucus, and potentially reducing viral load in the nasal cavity. For instance, a 2012 randomized controlled trial published in Family Practice found that patients using saline irrigation reported milder symptoms and fewer days off work compared to controls. The mechanism is largely mechanical, not antiviral or antibacterial, but it can facilitate better mucociliary clearance and reduce congestion.

However, while saline and seawater-based sprays are often used to prevent or relieve cold symptoms, there is limited high-quality evidence to suggest that they can actually prevent colds outright. Most studies report symptomatic relief rather than true prevention. Nonetheless, the use is generally considered safe and is supported by clinical guidelines, making it a scientifically validated adjunct, though not a definitive preventive measure. The evidence is not strong enough to warrant a higher rating, but it is more than purely traditional use.