Hi Steve,
I new to the U-boat modeling game and I myself am still getting up to speed on all the details...with that said, the cables in question I believe were attached on while out to sea and off while in port, pretty sure I read this somewhere. You just have to decide which setting the model is in. As for making them. I would say really thin gauge copper, somewhere in the 24 to 26 gauge range, try even 28. Take three or four strands, put the ends in a variable speed drill on one end and clamp them on the other end in a vice or with vice grips to your work surface. Try to keep the clamped side slightly separated, about an 1 in. (25.4 mm) apart in this configuration:
X
X X
X
Slowly turn the drill on keeping the wire fairly tight. The wire will begin to wind up and appear in the cable fashion. You could experiment with the end of a pencil where all the threads of wire come together as you wind. By sticking the eraser end in to fan out the wire slightly allows the wire to lay more concentric around itself. Google "Rope-walk". There are small jigs you can actually make, with scrap plywood and/or pine in about 1/2 hour to an hour and create some nice cable. Along with coils of rope for the deck and rope for flags.
Note : Use nothing more than 12" (30.48cm) The longer you go, the more chances of it breaking. Play around with it, Don't go too fast.
All said, you may even be able to use "Stranded" wire and just tighten the wire a little so it doesn't become un-stranded when forming it to your desired shapes.
Hope some of this helps. Good Luck!
Cheers!