Either way (birth-control pill or tampon) will work. I'm a scuba diver and I've done both for dive vacations. I prefer using tampon because it's a more nature way. Just remember you can always choose not to dive or dive less if you not feeling well. If you choose birth-control pill, you should talk to your doctor and may need to start it 1-2 months ahead of your vacation. You might still spot even if you take the pill on time. And there might be some side effects.

Sharks are always there. Ocean is their home. But I don't think they are attracted to this. There are some studies about it.