You don't join an alliance or the frequent flyer program of an alliance. Alliances don't have frequent flyer programs. Their individual members do, with provisions for earning credit when you fly with other members of the alliance they belong to. You have to join the program of a specific airline. That might be the program of the airline on which you take your next flight, or that of a different member of the same alliance. It gets complicated for at least three reasons:

1. While airlines grant frequent flyer credit for flights on other members of their alliance, they may restrict other benefits to their own members.

2. The amount of credit you get may vary from program to program within an alliance depending on which member airline you fly.

3. The amount of flying you have to do in order to qualify for awards or receive frequent flyer "elite" status may vary from program to program within an alliance.

You can spend a lot of time at sea when you're not on watch researching these things on FlyerTalk and via the airlines' own websites. You can also wait until you know what airline your next flight will be on, and therefore which alliance matters to you, and ask for suggestions here about which specific program within that alliance you should join.

May you have safe travels, both by air and sea!