4. There's no such thing as a "waterproof" or "sweatproof" sunscreen. Waterproof sunscreen is an urban myth.
When shopping for a sunscreen, the "waterproof" feature is somewhat of an urban myth. You should never count on sunscreen to stick to your skin after swimming or working out — plus, per FDA regulation, sunscreen can only claim water-resistance for up to 80 minutes before reapplication.
If you do go in the water, it's important to reapply immediately after you get out. If you plan to be in the water for longer than an hour or so — say, if you're surfing or long-distance swimming — wear protective clothing instead.