After an argument with an idiot, I'd like to clarify a topic for all...
Even a waterproof sunblock needs to be reapplied. All the waterproof label means is that you'll be protected for approximately 2hrs while you are in/at/on/around/near the water.
As soon as you get out of the water you dry off, which rubs off the sunblock.
You need to reapply after getting out of the water.
Don't believe me? Put on waterproof sunblock, wait 15 minutes (the amount of time recommended before going into the water) and then wade into knee deep water. Watch as your sunscreen washes away in the oil-slicked water.
Also, be sure to get a sunblock that is broad spectrum, meaning it blocks both UVA and UVB rays.
UVA rays cause aging effects.
UVB rays cause burning.
The SPF indicates the time a person can be exposed to sunlight before getting sunburn with a sunscreen applied relative to the time they can be exposed without sunscreen. For example, someone who would burn after 10 minutes in the sun would expect to burn after 500 min if protected by a sunscreen with SPF 50.
Thank you, and go forth and sun yourself!