The captain, who has had what the doctor calls a stroke, is bedridden and denied alcohol on his doctor's orders. However, as he tells Jim, he needs his rum:

I lived on rum, I tell you. It's been meat and drink, and man and wife, to me; and if I'm not to have my rum now I'm a poor old hulk on a lee shore, my blood'll be on you, Jim, and that doctor swab...

The captain is an alcoholic, so his statement means that his rum is everything to him: both nourishment and companionship. It's more important to him than anything else at this moment, He pleads with Jim to get him some rum, and he offers him a guinea if he will bring him a drink. Jim doesn't like being bribed, but nevertheless, he does bring the captain a drink, especially after the captain tells him the doctor said he could have just one glass. Jim states,

“I want none of your money,” said I, “but what you owe my father. I'll get you one glass, and no more.”

However, Jim's nursing of the captain is interrupted by the sudden death of his father, which for a time diverts all of Jim's attention in that direction.