you can go swimming the water will temporary stop your flow but only for a bit , if its a public pool or even a lake, ocean ect you should use a tampon just for sanitary reasons. If you have never used a tampon before it may feel weird at first but you get used to it pretty quick and then when you are done swimming you can go put a pad back on .
That's how I ended up using my first tampon too. It works great, don't worry!
Btw: Don't get a tampon that will swell bigger than you'd like. I darn near put finger prints in a porcelin toilet bowl trying to get the darn thing back out. Might be worth testing one in water first, to see what size it will go to.