---

I have the same model and year. I've found the seacock to be very stiff to operate. I would continue to exert as much force as your able which probably wont damage the seacock nor the through hull fitting.

If you're able to break the handle it should be no big deal.

Another approach would be to remove the hose from the seacock, and put some penetrating lubricant directly on the valve ball and let it soak for awhile. I have no idea what penetrant would work on the valve without damaging it but I suspect some WD40 might work. Before reinstalling the hose it may be worthwhile to see if you can operate the valve just a wee bit. If it opens unexpectedly a face full of water could result but you should be able to close the valve even if it's messy.