Photoshop Tutorial: Underwater Turtle
First select a cool image. I selected this turtle image. Duplicate the original turtle image and hide the bottom(1st layer). Now, you will erase the sky as shown in the images below. Keep only the turtle and the sand. You may do so by any method you prefer. You can erase the background by using masking, pen tool, polygonal lasso tool or any other method. Name the layer as turtle.
Open an image of the ocean.
Select the ocean image and place it over the turtle image layer. The ocean is smaller than the turtle so you must press crt+t and hold down the shift button to re-size the sea, making it bigger. Look at the image below.