Tips about how I made the Underwater Wallpaper (see wallpaper).
Start with blank document. Fill background layer with gradient going from light blue to dark blue.
In Layer effects, use very large (250px) black Inner Glow, with blend mode = overlay.
In new layer, select very large brush, set color to white and put a single dot.
Transform layer (Ctrl-T) to make dot width less and height more.
Move white dot to top of image (so that only bottom half of it is visible) and set its layer blend mode to Overlay.