Absolutely! So many girls wonder the same thing. There is no physical harm associated with swimming during your period. I remember feeling very anxious the first times I went swimming when on my period, and I even feel that way now sometimes. At times I worry that the string from my tampon will be hanging out of my bathing suit or that it will leak into the water, but I don't think this has ever actually happened. I make sure that I wear a swimsuit with fitted bottoms. Sometimes I will wear my cover-up and change into a thin pad when I am just hanging out poolside.
Yes, you can, but you should use a tampon. If you're just hanging out by the pool, you can use a pad, but I would wear shorts or a cover-up over your swimsuit bottoms. Pads are absorbent, and the sticky part will likely fail if you wear it in the water, so I wouldn't wear one if you're actually going swimming. If you've never used a tampon before, or you're nervous about putting one in, there's a video that shows you exactly how to do it. Or you can ask your mom or one of your friends, to stand outside the door and talk you through it. It's easy once you get the hang of it!
Absolutely! There is no reason to miss out on hitting the beach with friends just because you're on your period. For the longest time I wasn't comfortable with the idea of swimming while I was on my period. I would sit out during swim class in high school mainly because I didn't think you COULD swim when you had it. Later when I started using tampons, I realized that it was possible and then nothing could stop me from getting in the water when I wanted!