Seafarers International Union- SIU?

I'll be getting my Merchant Mariners Doent shortly, and I was wondering which union would be best to join. Can anyone tell me about the SIU? If I sign up at an SIU union hall in Boston, how good of a job can they do with getting me steady work? Also, how much does it cost to join the union and how have you're experience's been with them so far? Lastly, which are the best ship's to work on in respect's to overal comfort, quality of life, and highest pay? By the way, will being prior Marine and Coast Guard help me out on a ship? Thanks.