A union worker may sue both the employer and the union simultaneously. This is called a "hybrid suit," pursuant to Section 301 of the National Labor Relations Act.
The U.S. Supreme Court, in DelCostello v. International Brotherhood,462 U.S. 151
, 103 S.Ct. 2281, 76 L.Ed.2d 476 (1983) held that the six-month statute of limitations prescribed by § 10(b) of the National Labor Relations Act, 29 U.S.C. § 160(b), applies to hybrid suits under § 301 -- where a plaintiff alleges that the employer has breached the collective bargaining agreement and that the union has breached its duty of fair representation
There may be two issues here, which I cannot adequately address, but I can identify: (1) The registration of the ship that you were working on may affect the law under which your lawsuit would be brought, unless the shipping line has expressly consented to U.S. jurisdiction for the purposes of the collective bargaining agreement; and (2) The fact that you quit, could be used to claim that by quitting, the employer didn't actually breach any agreement to you, because, once you quit, you weren't entitled to the time off. Your rebuttal to this is that the employer had already breached the agreement by misrepresenting your rights to the time off, and so your quitting didn't terminate the contract as the employer was already in breach, and you were not in a position of being able to determine your rights until after you came ashore.
You will need a lawyer with both employment and admiralty law experience (a rarity, so be careful to check the references of whomever you interview).
For a referral, in California, see this link
Hope this helps.