Wet, white cotton is ineffective. To determine the SPF an artificial light is directed on two defined areas, one without protection and one covered by a predetermined amount of sunscreen. The number is a ratio comparing the time it takes minimal redness to develop on each. There is potential for wide variability between products because the tests are done by individual sunscreen manufacturers. Sunscreens come in a variety of vehicles.
Manufacturers try to balance aesthetics with resilience.
Waterproof screens tend to be greasy. This designation is supposed to indicate the sunscreen's ability to withstand four, 20-minute water immersions. Don't bet on it. Sunscreens are clearly useful to prevent sunburn. To be most effective, they should be applied to dry skin at least one-half hour before sun exposure.