I've been able to free some frozen cables in cars and bikes (though I haven't had to do a boat steering cable) . Try tilting the cable into a vertical position as much as possible, then running Liquid Wrench down between the sliding cable and the sheath. Then judiciously flex the whole assembly and push and pull on the sliding part. It may take patience over a period of days. Once you get a tiny bit of movement you're close to full success, as long as corrosion hasn't caused some of the strands of the cable to break inside the sheath (failure to move smoothly, once it's moving, might tell you that's happened). WD-40 would probably work as well, and it is less smelly than Liquid Wrench (to my nose, anyway). Either way, you'll have an oily drippy mess to dry out after you're done, but it may save buying a new cable.