I like the composition of this piece, but I also feel that the mermaids are not fully integrated into the scene. I would suggest showing a hint of their tails under the water, perhaps another layer set to a low opacity. For the one on the left and the one in the rear, a gradient layer mask would make it look as though the tail is gradually less visible. For the one on the right, the tail should fade out in between where it goes under the water. Also, the lighting of all three mermaids needs to match the lighting of the scene. The rear mermaid looks quite realistic, but the one on the left has strong shadows that indicate a different light source. Additionally, none of the mermaids are casting a reflection.
Also, pay attention to your depth of field. The rear of the scene has a very painterly quality where the focus of the camera drops off and makes the image blurry. Thus any objects you place in the scene should be more blurry the further they are from the camera.
Otherwise, the triangle created by the three mermaids creates a strong focal point for the image. The birds serve to draw the eye down into the image. The colours are also working well together.
Overall, I think that with some editing, this piece shows promise.