What's this movie?

I watched this movie years ago and would like to know the title of it, thanks. Here's what I remember:

A girl is in the woods next to a river, where she sees a young man kill a woman.

Time passes and the two meet and they act really weird as she hints at what she saw and he's tempted to kill her. They go to a hotel at one point, I think, but nothing happens.

The movie ends with the two going on a boat ride in a lake and she starts acting wild, standing up and shaking the boat while the guy is freaked out. I'm pretty sure she's suicidal. She dies by drowning.

The movie cuts to the boy crying saying he didn't do it and the police guy says he knows. I think the police guy knew the boy killed the first girl but not the second, but wants to use it to imprison him for the first one, which didn't have enough evidence to convict him or something.