Look for water-resistant products, though even these should be reapplied after swimming or heavy sweating.
Like all sunscreens, water-resistant products should be reapplied after two hours, regardless of water exposure, and after toweling off.
ASTM C 150 defines portland cement as "hydraulic cement (cement that not only hardens by reacting with water but also forms a water-resistant product) produced by pulverizing clinkers consisting essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulfate as an inter ground addition."
Very water-resistant products protect for up to 80 minutes.
