Help with a (rebellious?) teenager?

I certainly remember going through a phase in my teenage years where God knows what could be going through my head at any particular time. But then there's my daughter. She's the oldest of my 3. She's 16 now, and seems to do things like this just to spite me and her father. It started this summer, when she would lay out by and swim in the pool in the nude. The first time I saw her, I just laughed and figured she'd be embared and not do it again. But she did it more and more to where I finally had to go out and tell her to stop cause the neighbors might see. She said there's no windows above the fence. I told her, even so, she's not a little girl anymore, and its disrespectful to let her father see her that way. So I said, "As long as your father and I are home, you have to wear your swimsuit." It stopped for about a week. I threatened to ground her when it started again, but she always says "sorry I didn't know you were home." or makes up some sort of excuse (she's does print modeling as well, and claims her tan has to be perfectly even). The summer ended soon enough, thank God, but the problem didn't stop. Then she began to sleep naked, and consequently come downstairs to grab her midnight snacks or her breakfast in the nude. It's not something you WANT to deal with - bumping into your teenage daughter naked in the kitchen - so i always just look away and try to let her lose interest in it on her own. One final part of this - I know she's a teenager, and I know teenagers masturbate, but I have accidentally walked in on her several times in just the last couple of months, and I have good reason to believe that it's occupying 4-5 hours of her life every day. How do I approach this situation. I'm in over my head.