I think square acres might work as the article could be referring to the area moment of inertia of the floating vegetation (https://en.wikipedia.org/wiki/Second_moment_of_area). I suspect this is not the intended meaning though. :)
That's because a fathom is a unit of length (or depth) so 4 in to 1 obviously isn't going to work.
What does work have to do with any of this? Sure, the water acts on the floating vegetation in the direction of displacement, but without a measure of the weight of the islands, it's somewhat meaningless.
4D: Volume and time! Obvious!
The demonstration is trivial and is left as an exercise for the reader.
Of course, but what is the conversation factor between seconds and feet?