small diameter (undersized for the job) 30' rope.

Question: When the rope snaps, which way will it go?

Should it snap back towards the boat in tow (the load). Or should it snap
toward the boat pulling the load (the force)?

I was always taught that you look at the worst case first, in other words
the rope snaps in the center. And in that case the ropes will snap to their
respective tie points