The SPF claim on a sunscreen label is an indicator of how much time you can be in the sun until your skin begins to get damaged or burn. SPF is a multiplication of your average burn time (without sunscreen) to provide a reasonable indication of extended time in the sun before burning with SPF factor applied.
So if you are fair-skinned and start going red after only ten minutes in the sun an SPF 30 that keeps working even after swimming, will provide up to 300 minutes of effective sun protection. That's five hours of sun protection which is normally plenty, especially when combined with sun-smart behavior. If the sunscreen you're using comes off the skin due to water, sweating, or rubs off you may incorrectly believe you are protected due to the high SPF promise you read on the label when in fact the screens effectiveness is significantly diminished. This is why it's important to re-apply after sweating heavily, prolonged water exposure, or towel drying.
However be careful, you can't expect to expose skin to the sun all day just because you're wearing high SPF sunscreen. Put on a shirt or head to the shade well before you reach any mathematical burn time.