A few weeks ago we were at the beach, where we took a bunch of pictures (of course.) Here are a few that didn't turn out so well.
This one has a couple of problems. First, and most obvious, it is focused on the water instead of the subject. This is a no-no. You can read about how to focus your camera here. Second the frame cuts off part of the subject - her toes. There's no rule that says you can't ever cut off part of your subject (in fact I talk about it being good here,) but in this case, it hurts the image because it seems accidental and brakes the frame awkwardly.
This picture doesn't have either of the problems of the first image, but it is overexposed. Notice how the shoulders are "blown out," and the pink swim suit looks almost white in some places.
This last shot doesn't have any of those problems and is a much more successful photograph.