How do you unfreeze a steering cable on a boat?

Disconnect the cable and roll it up and immerse in diesel fuel for a few days this will help lube and break up the rust. You can add a couple quarts of Transmission Fluid to the diesel it will help slick things up. After a few days gently try to pull on both ends to see if it will move. If it does soak some more and keep trying till its free. Hope that the corrosion hasn't eaten enough on the cable that it will snaps if you put a load on it.