I have gone to camp since I was small, about seven years old. It was mandatory to take swimming. The instructor was a nice man, about my dads age, who liked me a lot. [I was his favorite] As I grew, I continued to be his favorite. He used to always take me in a boat across the lake, and he would always make sure I was having fun. When I became a teenager, I was still his favorite. I would be with my friends, yet he would only say hello to me, and he would ignore my friends. In the past two summers, things have gotten creepy. It started with him only saying hello to me. It then progressed… In the winter I saw him at a party. He asked what school I went to. I told him a private catholic school. He said "Oh your smart and beautiful too?" I thought nothing of it, as it was a genuine compliment. He then asked,"Do you have to wear the uniform?" [skirt, knee highs, etc.] I said yes. He said," Do you roll your skirt up so the boys can see your underwear when you walk up the stairs?" I replied no and got away. This summer, he has constantly been asking me why I am not swimming, and asking why I will not put on my bathing suit. I say "Idk" and he keeps saying to go get my bathing suit on and go swimming. He said, "see you just sit there, and you’re going to get fat." He doesn’t say this to any of my other friends, it is only me. On Friday he asked if I was going swimming. I replied ‘no’ and he said, "you used to love to swim. What happened?" I replied, "I don’t know." He said, "I’ll tell you what happened. You went through puberty." I was mortified. He said this in front of other people who were disgusted. It’s awful. I feel so uncomfortable. It’s a very close knit camp, so I really don’t want to talk to the director, and cause trouble. I told my dad about this last year, as it was going on then, but I don’t want my dad to handle things…Because what dad is going to like a man his age saying that to his teenage daughter..I know this lifeguard is going to continue saying these disgusting comments. Do you think I should say something to him? If so, what should I say? I don’t want to cause trouble, but I don’t want this to continue to happen either.
I don’t like it. I hate it. I dread having to see him.

You need to talk to your father and with him approach the camp director. A man like that should not be permitted to be around children and teens. Right now he is just saying inappropriate comments but you don’t know if he may wind up going further.

I suspect this behavior was going on when you were younger but you just didn’t notice it. This isn’t just about you, when you no longer go to that camp it will be somebody else. This is not a situation a teenager should/could deal with on their own. Even though you don’t want to cause trouble, the adults need to be informed and need to step in. In this situation your not the cause of any trouble, this disturbed older man is.