I like how you created this fantasy environment using the mermaids and the way you positioned them is nice on the rocks, the appear like they are actually sitting there. You could make this even more convincing if you added some shadows where they are sitting by using the burn tool on the background behind them. I like the addition of the planet in the sky it coordinates in color well with the rest of the background, but the way that it was added to the image could be improved. Notice that there is a white ring around the edge of it, that is somewhat distracting, but this could be fixed and be made to look more natural and not distract from the other elements in the image. You can fix this by adding a small inner glow and a larger outer glow on the layer that the moon is on, this can be done by just double clicking on the layer and then a new window will come up and you can apply these glows from that menu. Hope that was helpful, if you ever need any help or suggestions/critiques on anything, please ask