For starters (as Rod points out), your image mode is greyscale, trying changing the image to RGB mode = Image/Mode/RGB.
In the link that O provides, remember that you have to use a selection when having the bottom half of the image being the water scene.

You see the "New Layer" layer? It has a mask (border in black) and the gradient application (border in white). Those two borders can be selected, so when you decide to color the bottom half of a water scene (following the tutorial link that O posted) you would select the white border area of the layer mask, with the bottom half of the image selected, and then use color gradients (foreground/background) of light and dark. I strongly recommend that over using colorize, which just makes the color one flat color. Using gradients gives the colors depth, like we actually see with our own eyes.
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"In order to attain the impossible, one must attempt the absurd."
~ Miguel de Cervantes