The S.S. St. Louis was a ship full of unscreened potential refugees with unproven economic potential coming from a war zone filled with enemies of Canada and the United States, but seeking refuge.
We see the rejection of the St. Louis' would-be refugees as a tragedy, but by the standards many people advocate for determining which refugees we should accept, we would turn it around again today.
Rules should be general, not specific. Sometimes our general rules will not give us the specific results we want and bad things will happen. We'll let in a criminal, a spy, or a radical, or we'll condemn innocent people we could have saved to die. Perfection isn't an option, and neither is a costless policy.
Whether or not we should have a rule that would require us to turn away the St. Louis today is arguable, not a discussion beyond the pale. But people who advocate for that rule should have to own up to the potential costs of their preferred policy, not just preach the virtues of caution.