Flotsam is one of the weekly Legion World Bosses in World of Warcraft. You need to be level 110 and have the World Quest called A Dark Tide active yo be able to get loot from him.
In order to get to him, take the flight path to Shipwreck Cove in Highmountain. It's the flight point in the far north. Once you are there, go northeast for a minute and you will reach him.