I had to rotate the background picture 90 degrees counterclockwise to position it right.
Picture of boat:
I used scissor in toolbox to cut it out and copied it into the background picture. The boat is slightly rotated to visualize the weight on one side. Furthermore the boat was shrinked to a smaller scale.
Picture of a boy:
I cut out the body, shrinked it and copied it into the boat. I used the paintbrush to soften the edges. I placed the boy at the tip of the boat, to make it look like it was almost sinking.
This is the final result! I used opacity and colours in the toolbox to darken the skyline and water. I also highlighted the edges of the boat, to make it look like the sun was reflecting the light. I kept the boy oversized to add tension and focal point in the picture.