To make this case a lawyer would search through 20 years of union records: pronouncements, advertising, newspapers/newsletter, negotiations, speeches. In all of that did the union ever claim to make the work place better by changing work hours, living conditions, liberty rules, etc. ? if they did then the lawyer would argue that the union is equally responsible along with the employer in creating the conditions aboard, and therefore equally culpable.
And as @jdcavo has pointed out, if the plaintiffs lawyer cannot prove the vessel unseaworthy, they are going to try to find restitution some place other than the employer. And if the plaintiff has sufficient animus, that may be the union.
Another line of reasoning would be this: the lawyer could make the claim (entirely without merit) that union members have a longstanding tradition of never ratting each other out, and that this unwritten but ironbound code fostered the conditions for the crime to occur.
The lawyer would make some story up ( fantasy of course) that when officers (employers ) ask union members if they saw a fight or an accident aboard ship that union members have an unwritten code to say they never saw anything, and that this unwritten code fostered an atmosphere of impunity that lead to the crime.
Fantasy of course, but plaintiff lawyers win cases not so much as by setting a fire as stacking the cordwood high and threatening to light the match.