Re: dangers of a steel cable in hand winch....unfounded?
For your rig, there's nothing to worry about (try to discern "probable" from "possible.") Just watch for rust through, which takes years of salt water use. Even if it breaks, it's not going to whip around, slice your head off, break your rear windshield and cut your hull in half. Worry about that when you get the 25' boat. Small boats like yours are not a problem.
I have used "wire rope" for decades in salt water applications where boats are winched on, not driven on. Nonetheless, I don't stand directly in the line of fire when I'm winching. I'll admit I recently replaced a rusted wire rope with a strap, mainly because it was cheaper, and for this trailer, I drive part way on (until I get my power winch).
As for length: You back the trailer in so the axle is an inch above the water. Unlatch the tilt latch. Pull the boat against the trailer; it will tilt down some, pull it in tight. At this point your bow eye is close to being over the axle--and that's as long as you need the cable. But may as well buy one that's 15'. Point is, you don't need one longer than the boat b/c you are always starting with the boat at the trailer.
The only problem with getting one too long is that you lose the efficiency of the winch as cable/strap winds onto it. IOW an empty winch spool cranks easier than a half full one--so try to start close to empty.