What to do - 4th of July period struggles

So, tomorrow is the Fourth of July, which basically means swimming the entire day at my grandparents lake house. But I got my period today. I have a friend who is sleeping over tonight, and I tried putting a tampon in tonight twice, and I got it in but it was really uncomfortable. We went out on a quick boat ride but decided not to go night swimming like we planned. I got the tampon out but It still feels really uncomfortable 30 minutes later.

I feel really bad because I want to be able to swim with my friend tomorrow, but It was so uncomfortable and I don't think I'm ready to try again. Tampons are no big deal for her, she wears them all the time and has for years. I feel bad because I'm older too and I can't do it.

This kind of thing has happened before where on a day we went to a theme park, i couldn't do any water rides and she didn't do any either because of me, but was understandably kinda disappointed.

I want her to be able to enjoy the day and not feel guilty for swimming if she wants to, and I don't want to have to like conspicuously sit out from everything but I don't know what to do...