As far as I know, there’s only one, true “union” for deck officers: MMP.
Someone already mentioned AMO’s hiring process: you check an internet board; email that you’re interested; they tell the company; company hires you (or doesn’t). That makes AMO a crewing agent, not a union. The company MIGHT hire you… they might also hire another 3/m’s cousin/brother/friend. You won’t know. And why give the company that power anyway?
MEBA, is a marine ENGINEERS union. They have deck jobs, but you’d be a carpenter in a plumbers union—an afterthought at contract time (and every other time…).
I’ve been MMP for 16 years. I’ve never had real trouble finding work. I took low paying jobs, sure. But I’ve been on Matson ships, too, and permanent for the last 8. What I tell everyone who asks is this: MMP is without question harder to start out. BUT, once you get a foothold, it’s not even a debate—it’s worth it. Be ready to take jobs you ordinarily wouldn’t, at times you don’t want to, and you’ll never be out of work, gaining points and seniority the whole time, so you can get to the place where you can pick and choose.
And on the hiring hall… no one forces you to go (it’s not the MSC “pool”) and you don’t have to sit there all day. It’s wise to go in often, starting out. Then you learn how to handicap. Besides—the hall is kind of the point of the union, isn’t it? You’ll know who got the job, and why. You can challenge their qualifications. And the union decides who goes to work; not the company—they don’t get to choose their buddies and ass kissers. It took 30 years of fighting to wrestle that from the company—embrace it. Enjoy it.