The cleanest sea in Spain is consistently found around the Formentera and Menorca islands in the Balearics, specifically within the protected waters of the Ses Salines Natural Park. Formentera is world-renowned for its "Caribbean-like" turquoise waters, which owe their incredible clarity and purity to the Posidonia Oceanica seagrass meadows. This seagrass acts as a natural water filtration system, oxygenating the water and trapping sediment to maintain high levels of transparency. On mainland Spain, the province of Almería, particularly the Cabo de Gata-Níjar Natural Park, is frequently cited for having the most pristine coastal waters due to its volcanic origin and lack of heavy industrial development. Furthermore, Spain consistently leads the world in "Blue Flag" certifications, with regions like Galicia and Valencia often receiving the highest number of awards for water quality, environmental management, and safety. For 2026, travelers seeking the absolute "cleanest" experience should head to the beaches of Migjorn or Ses Illetes in Formentera, where visibility often exceeds 30 meters, making it a premier destination for sustainable tourism and marine conservation in the Mediterranean.