How can I hide self-harm scars at a water sports trip?

If your cuts/scars are low on your wrist, perhaps you could cover some of them with bracelets or armbands of some sort? It's not ideal, but it could help. Also, if you have a foundation or concealer that's oil-based, it will actually do surprisingly well in water; it would be worth a try. Depending on how concerned you are about people seeing your scars, you might want to consider just wearing long sleeves in spite of your teachers' wishes. It sucks, but I can't really think of any other options. I know how stressful it can be trying to hide self-harm in the summer; I wish I had a better answer for you :( But at the end of the day, even if you can't keep it completely covered, it's not the end of the world. Often people who notice scars and cuts will just pretend they didn't, and usually if they do say something it's just because they're concerned and they want to help you. If anyone gives you shit about it, that's their issue. Do your best not to worry about it. I know it's scary, but stuff like this is almost never as bad as we think it's going to be. I wish you the best of luck :) I'm here if you ever need to talk.

Why hide them. You made them, they are part of you. You do it for a reason, so accept the fact that others will see them for what they are.