Types of Images
The images created by flat mirrors and underwater are both virtual images: the rays of light appear to come from the image, but in fact they do not.
There's another type of image, called a real image, where the rays of light really do all pass through the image, even though the image is not the original source of the light. Real images are created when the rays of light from the object, which are originally diverging, are caused to converge to a point by some device. An observer who is able to see the rays after this, will trace those rays back to a point and see an image there, but unlike in a virtual image, the rays of light really did come from that point.
Real images have several distinctive features compared to virtual images. First, you can project real images onto a screen: place a piece of paper at the location of the image, and you will see the image there. Cameras create real images on film or chips, our eyes create real images on our retinas, and so forth. Conversely, if I place a piece of paper behind a flat mirror, I cannot see a projection of the things I see in the mirror.
Also, notice how the rays of light cross after the image, with the top ray going to the bottom and the bottom to the top? As a result of that, real images are inverted as compared to the object, as if they were rotated by 180°. This is a signature of a real image; virtual images are not inverted. if you've ever looked into the bowl of a spoon and seen yourself upside down, that was a real image created by the spoon. (Notice one implication: your eye projects an upside-down image of the world onto your retina, and it's your brain that flips it back around again!)