One thing you have to take care about is the position of the scene light that creates your ground shadows - it should more or less match the position of the light in the background image.

In your photo, the sun is high and coming from front right (notice the shadow of the boat's prow on the right, and the balcony railings on the far left). But the scene light is set low (and further back on the right), so the submarine's shadow is long and the ground underneath it is fully illuminated. That shadow doesn't fit with the background.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)