Go to any tavern where you can hire a second mate. Barter with him to reduce his asking price. When he lowers his asking price, choose the option to reject the offer anyway. Keep doing this, even if he doesn't lower his price...just keep doing it. Eventually you will lower the price to 0 or darn close. The amount you reduce the price each time is your Commerce Skill x 100. So a skill level of 5 would be equivalent to getting a 500 gold reduction each time you barter. Eventually, if you keep this up you will get him to start offering you negative gold in exchange for hiring him. Do this as much as you like for as much gold as you like, I have not found a limit yet!