Why was the male slave forced to drink rum?

It's a very crude way of preventing the slaves on board the ship from dying. Gardere tells Jessie that he's often seen slaves die through sitting down, with their heads on their knees, and not moving. So according to him, you have to keep the slaves distracted, get them moving, otherwise they could easily die. Forcing them to drink rum is one way of achieving that. Jessie doesn't have the faintest idea of how slaves would die from just sitting down in a certain posture. Gardere doesn't have much of a clue, either, but he swears blind he's seen it happen before with his own eyes. In any case, what this episode shows is that saving the lives of slaves on board ship is not motivated by any humane considerations; it's simply a way of protecting a very valuable cargo.