Use layers and masks to create this awesome composition!
Place your image of your fish in an underwater environment. Grab the Quick Selection tool and select the fish. Go to Layer>Layer Mask>Reveal Selection. Now click on the layer mask thumbnail and go to Windows>Properties. Set Feather to 2 pixels.
Add a ship in a bottle. Grab the Pen tool, and then create a path around the bottle. Click Make: Selection from the tool option bar. Now go to Layer>Layer Mask>Reveal Selection. Adjust the size to fit in the fish’s body.
Move the Boat layer under the Fish layer. Grab a soft tip brush, set the Foreground colour to black and paint over the mask to hide the body. Lower the brush Opacity to create a semi-transparent area over the head.