Sunscreen is the most important thing that you can ever wear outside. Depending on the SPF that you're after depends on how much coverage you're going to want when you go outdoors. When it comes to limiting yourself to putting on sunscreen just for when you're swimming, you shouldn't because it's best to keep it on any time that you go outside. Sunscreen is used to protect UV rays from entering your skin and preventing the chance of sunburn on your skin. Making sure that you reapply your sunscreen is just as important as first applying it. Depending on how long you've been out in the sun and what the number is on your sunscreen bottle corresponds to how and when you should reapply. If you have a higher number, then that means you have to reapply less.
What's also great is there are a fair number of sunscreens that are waterproof and last longer while you're in the water. While there is a conception of not needing sunscreen while you're in the water, UV rays can still penetrate through the water that you're swimming in, so putting on sunscreen is just as important as your suit. See how long you're going to be outside for and plan how high you need your SPF to be. SwimOutlet has a wide variety of sunscreen ranging from 15 SPF to even higher. If you need something more concentrated then there is even zinc available for the lifeguards and surfers. As long as you're wearing sunscreen you should be well-equipped to tackle any outdoor activity.