Not sure what the laws/morality of the city in which this bar is located in is...but...perhaps hanging cages that hold slaves to be sold? Or...to hold those that got out of line to serve as an example to others that might cross the gang that owns the ship. Or..if not hanging cages, perhaps cages that are tethered to the side of the ship in the water kept afloat by a barrel or something.

Also - not sure if this can really be depicted - but perhaps an escape hatch in the lower levels so the owners could swim away if the heat was on...or if there is a hole in the ship, maybe a little dock that can only be accessed by pushing a small boat under water and up into the hole.