Mawapi-Addon is a small bandit hideout on a cape between Alt Bosara and Tel Mothrivra. To find the cave, head along the southern part of Lake Boethiah's eastern shore and keep your eyes open: the entrance is set a short distance back from the shoreline, amongst the rocks at the feet of the hills.
The smugglers don't seem to specialize in one particular type of loot. There are crates of skooma, moon sugar, Dwemer artifacts, kwama eggs and other miscellaneous items. A few bottles of drink can be found, and a scroll of Battlecry, which teaches the spell Blood-letter when read, make up the rest of the stash.