Dropping either brick into the water will result in a net lowering the water level.  But dropping the iron brick lowers the water less, thus leaving the water higher than it would be if we had dropped the gold brink into the water. Iron makes the final water level higher.  

While in the boat, each brick displaces an amount of water equal to its weight, and then when dropped-in, displaces and amount equal to its volume. Since the bricks are denser than water, the first displacement (downward) is larger than the second displacement (upward). 

While the brick is in the air dropping, the boat rises up and the water level falls. Then, when the brick submerges, the water rises back up, but does not regain its original level.

With the gold brick being the heavier brick, the initial water level fall it causes is greater. The rise back up, when each brick submerges, is the same for each type of brick, since they have the same volume.

(density gold = 19.3 gm/cm^3, density iron = 7.87 gm/cm^3)