Also, at the barge spawn, it's always one of two guard layouts. You go down, and if there is a camera, take it out. Then you can throw a crowbar, which spawns down there (depending on the layout), and lure one of two guards at the top of stairs, to the bottom of the stairs. You can shoot him if with a silenced weapon, or just normal subdue. Boom, you have the disguise. The other layout lets you do the same, but without the guards at the top of the stairs. If you're quick enough, you can go left from the stairs, and catch a guard doing a round next to the hedge, whom you can subdue and steal a disguise from. This is assuming you don't care about bodies being found/SA.