Only once a game, sandbox only, great. :) Also, the person who's getting ferried should decide whether to leave or not, otherwise it's just a troll-role.
The Boatswain player's primary incentive to ferry will not be to win as a third party; it will be instead that he is a terrible person if he deliberately chooses not to take advantage of the best role in Epic Mafia. In my judgment, this is an offense worse than game throwing and should be punished by...ferry.
Also, the person who's getting ferried should decide whether to leave or not, otherwise it's just a troll-role.