For starters, never approach the cities water tower. It's set in the middle of a pasture of sheep destined for the plates of rich Luskanites. You can imagine what happens after they're done asking you questions. Also, you'll want to avoid the Whitesails Harbor unless you're a member of the Luskanite navy. You look more like a land lubber to me so stay away from there.
Guards have orders to kill on site anyone without proper authorization.
They are even rewarded for each body they bring in. The area is mainly comprised of warehouses for the trading companies who reside in Luskan. Aside from the warehouses there is the North Gate which is always well guarded and usually closed at night. During the day, you can often find beggars and the like huddled around the gates. I suppose one could use them to their advantage if one didn't want to be seen poking around Luskan asking suspicious questions.