Camping with a friend, doesn't know about my SI?

[brain child of boredom]

So my best friend and his family are going camping with a few other people this Sunday into Monday, and they invited me. At first I had told him that my mom wasn't really in a good mood (which was true) so I was afraid to ask, but he said he really hoped I could come, so I felt guilty, so of course, asked, my mom and his dad talked, so now I'm going. My problem is, I have scars from cutting up and down both of my legs, and pretty much nobody knows about them. It gets even worse when the fact that the main activities to do there are hiking (more than happy to do), canoeing, and swimming, and for the last two, it isn't really practical to wear pants, and there is no doubt that they will want me to swim since we used to hang out at the public pool a lot. I am really self conscious about the scars, and a lot of them are obvious, and not just to me, so i can't even go with wearing the boy-like board shorts, especially since it's worse on my lower legs, so I am really quite anxious about going, especially since I'm not exactly ready for anybody to know what I used to do, and still sometimes want to. I know that they would be quite accepting, but I still can't bear to let anybody know, what can I do??
Thanks to the two who have answered so far. I should clarify though, they are scars, not recent ones, and I'm having trouble thinking of excuses, I've used pretty much every one there is to be used (that I can think of)