The words, flotsam and jetsam, imply contempt for the people. It sounds as if they are considered trash, waste, discards. In other words, debris. If you have any sympathy for these people you would not use the terms. And I'm not sure running is and action applicable to flotsam and jetsam. Don't they just sort of lie there?
Also, "flotsam and jetsam" is a bit of a cliche. Do you really know what each word means? Do your readers? Or is it merely a vague term with a hazy image (at best) evoked. This is what a cliche is. It's a phrase so overused it loses all impact and nearly all meaning.
I agree. After all, if only the flotsam and jetsam were there, what happened to the lagan and the derelict? They're more likely to stay where they were than the flotsam is, so the metaphor is wrong anyway.
