Hmm... a desert dragon, and a high-sea robber. Interesting.
Though, with an underwater lair, it seems like you have a sea-faring theme on your hands.
One idea: Ships vanishing at sea without a trace. Local economies and perhaps public health are suffering due to the loss of food and trade goods. This could appeal to the Brass Dragon's inherent goodness, and a pirate's desire to keep things stable in his watery domain.