What does it mean by Drink and the devil had done for the rest?

The verse from the pirate song in which this line is found (and which was created by Robert Louis Stevenson for his book Treasure Island) begins, "Fifteen men on the dead man's chest". So the subsequent line in the verse ("Drink and the devil had done for the rest") means that, aside from these fifteen remaining living men, all the others on the ship's crew had died, either from drinking or as a result of various evil acts or misfortunes (that is, acts attributed to the Devil). "Had done for the rest" means that these evil acts or misfortunes had accounted for the dead men's fates.

UPDATE: "Drink" or "the drink" is a sailor slang for sea, so "drink" in this case probably refers to the rest of the men drowning, not perishing from drinking.