While you are swimming outside, you need to protect your skin from the sun's rays - both UVA and UVB. There are a multitude of sunscreen and sun block products that can do this; how well they work, and how well you like them is going to take some trial and error on your part.
The first thing to consider is SPF (Sun Protection Factor). This gives a numeric value to compare one product to another. The SPF tells how much longer you can stay out before burning than when you are not using a sun protection product. This does not mean that you should stay in the sun longer, just that you are getting more protection from a higher SPF compared to a lower SPF.
Next, you have to consider your skin's sensitivity to the product. You could be allergic to some of the chemicals in the product you choose; one of the popular chemicals, PABA, does cause a reaction in some individuals; if this is true for you, then read the labels carefully and choose a product that is PABA free.
What about waterproof or water-resistant products? Water-resistant products must maintain their SPF after you have been in the water for up to 40 minutes. Waterproof products must last up to 80 minutes.
Practically all products will get wiped off when you use your towel and must be re-applied. To protect your eyes, use a good pair of UVA/UVB rated sunglasses. Add a hat to protect your head when you are out of the pool. Remember, most experts recommend an SPF of at least 15, and you must re-apply the product after each swim for the best results. Always read the label before you buy it.
Good luck, don't get burned, and Swim On!