I just did this quest 10 minutes ago, its a chain quest at the end. Basically, you talk to Marshall thingy and start it, the lighthouse is lit and the skies go dark. Archers line up on the docks and a huge creature appears. Whoever is doing the quest can  right click the cannons to fire cannonballs at him. He will use a  knockback ability all the time, so you'll just have to run back to the edge if you get knocked back. He doesn't come out the water and he doesn't melee. If you go into the water and attack him with melee, he still won't hurt you. He is a lvl 100 elite beast. The rewards for this quest are really good. *nod*