It works... all day long! Doc Martin's sunscreen has a distinctive delivery system unlike any other sunblock that provides an SPF of 36 protection, feels cosmetically elegant, and lasts ALL day in water. This Sunscreen gives both UVA and UVB protection all day. After 6 hours in the water it is still an SPF 20! Perfect for surfing, swimming, hiking, running. Dr. Martin has worked with other surfers, windsurfers, swimmers, cyclists, tennis and golf players in product development. The results represent some of the most significant advances in sunscreen technology in the last years.