Spectra (low-stretch) rudder lines are an alternative to wire cable.
Spectra line is also a possible addition to the emergency repair kit.

Stellar sells Spectra line in two diameters.
I really like the orange line on the Think surfskis (it is stiff - good for pushing through under-deck tubing, and has a nice waxy feel and holds knots well), and some online searching indicated that it was Q-Powerline, used for kiteboard rigging. I contacted Brian Smith at Powerline and got the following info:

The cable we make specifically for the surfskis including Think is different than all the other lines we make. It has a special core and a special proprietary polyurethane topcoat. It is not the same as the similar lines we make for spearfishing or for Kiteboarding.

So, if you want that line, you'll need to contact a surfski dealer. Minimum quantity from Powerline is a 3000-foot spool.. ;-)