Advice please?!

So I am 14 years old, and in the holidays my dad and stepmom are taking me and my sisters in a cruise. 
I use to self harm,and my family knows this. But, they only think it was on my arm (and not my thighs). I have the worst of my scars on my thighs which have continued to remain purple for months. I really do not want my family to see them, because they don't cope or 
communicate well with the situation and it causes avoidable arguments an drama. 
The weather will be warm, and we will be snorkelling (my bathers are bikini, board shorts and a rash shirt). I will be very obvious if I don't wear shorts.
What are some ways I can cover my scars. Or should I just sit my family down and tell them.
Please no judgement, thank you