Margarita,

Straw is an archaic verb meaning to strew; there is a river Swin in Belgium, but I think the most likely one meant is the shoal ground of that name S of Harwich. I don't know of a shoal called The Ducie, but I suspect that that is what this is also. "Blue lights" were pyrotechnics carried by merchantmen as distress signals. The ship in the poem is aground and calling for help.

Pete M