Now if you remove the ski pylon it will upset the natural balance of things. The boat will get confused and will most likely start crabbing sideways!

I think you have a bit of corrosion going on there, if you don't mind damaging the pylon then I would suggest a BIG pipe wrench to twist it and break it free. If you can find a big strap wrench you could also try this without it marring the pylon. Also try shooting some PB Blaster over that area, this stuff is great at freeing up corroded parts. I had an old MasterCraft and we could never keep the thing tight, I was always down there tightening those set screws!!