To find the distance between the seagull and the large fish, we can add the height of the seagull above the water to the depth of the fish below the water.
The seagull is flying 4.5 feet above the surface of the water, and the fish is swimming 1.5 feet below the surface. Therefore, the total distance between the seagull and the fish is:
\[ 4.5 \text{ feet (above water)} + 1.5 \text{ feet (below water)} = 6 \text{ feet} \]
Thus, the seagull and the large fish are separated by 6 feet.