Okay, this was really dumb. Once it tells you to find an alternate route to the docks, just hug the wall on the left to go behind the two guards. You'll pass a dead woman on the ground back there. Walk a little further and you'll notice that you're now on the lower level, kind of under the two guards that you just slipped past. Check your camera-you'll see two heavies on the right and then a few other baddies on the left. Now, stay to the RIGHT and sneak in between the first two guards that you snuck past already and the two heavies. Don't even worry about the other bad guys on the lower level. Anyways, do this and you'll get there no problem!
I won't give away the rest of the mission, but that should help you a bit!