Just spent an hour and a half identifying a weed while watching Voyage of the Mimi on youtube. There are several of these plants in my porch box and I wanted to know what they are, so I picked one (carefully, having discovered the other day that they have spines). I jumped on google and hunted up a weed identification site. After taking my best guess at some of the options, and viewing the pictures, it seemed to be a pigweed variety. A bit more image searching and spiny amaranth seems to be the most likely. I have no idea how it got in the box, and I hunted around the yard for a plant that looked just the same, to no avail, but that's what it is.
This genius moment came at the same time that the Mimi's crew, shipwrecked, set up a plastic sheet as a still to turn sea water to drinking water.