Did you ever wonder how the SPF is determined? The SPF number is determined experimentally indoors by exposing human subjects to a light spectrum meant to mimic noontime sun when the sun's rays are at their most intense. Some subjects wear sunscreen and others do not. The amount of light that induces redness in sunscreen-protected skin, divided by the amount of light that induces redness in unprotected skin is the SPF. It is a measure of UVB protection that ranges from 1 to 45 and above.
In other words, a sunscreen with an SPF of x allows you to stay out in the sun x times longer without burning.
A higher SPF doesn't indicate superior sun protection. An SPF 2 protects your skin just as effectively as an SPF 30. However, an SPF 2 will need to be applied more frequently because it's only doubling the amount of time you can stay in the sun before burning. However, both SPF 2 and SPF 30 need to be thoroughly applied and reapplied when the protection timeframe runs out and after swimming or sweating.
For a sunscreen to be marketed as 'water resistant', the time required to burn must be the same before and after two consecutive 20 minute soaks in a Jacuzzi (that part of the test I'd volunteer for). The SPF factors are calculated by rounding down the time required to burn, but I suspect you get a false sense of protection from an SPF because the amount of sunscreen used in the tests is a lot more product than the average person uses. The tests use 2 milligrams of formula per square centimeter of skin. That's like using a quarter of an 8-oz bottle of sunscreen for a single application. To put that in perspective, even though I go to the beach about every other day and always apply sunscreen, I go through about a bottle of sunscreen a month, not a bottle a week. Still... a high SPF confers more protection than lower SPF. Maybe with typical real-world use, an SPF of 30 is like SPF 15 and SPF 15 is like SPF 8.