If you're on your period, you should definitely use a tampon. However, if you're talking about normal discharge, I probably wouldn't. Here's why:
- Swim-suits have gussets, so it is highly unlikely that anybody will notice anyway. You may be self-conscious about it, but it's highly unlikely that anybody will even be aware of it. How do you think the other girls will manage? I highly doubt they're all going to wear tampons!
- Tampons aren't supposed to be used for anything other than soaking up menstrual blood. There's a warning on most boxes about this. However, if you've ever not been sure whether your period has finished and have used a tampon just in case, then you'll know that it doesn't really do you any harm. You should probably listen to the advice on the tampon box rather than me, though — that warning's there for a reason!
If you do decide to use a tampon, make sure it's one of the smallest, least-absorbent ones you can get. Put it in right before you put your bathing suit on, and take it out as soon as you're back. This should minimise any risk of danger.
I'm saying this not because I think you should do it, but because if you're going to do it anyway, it's best to be safe about it!
I honestly think you've got nothing to worry about, unless you have extremely heavy discharge. If this is the case, it might be something you want to talk to your doctor about, just to make sure everything's okay.
Good luck, and don't worry too much — you don't want to let your anxiety ruin your enjoyment of the activity!