Im running an unmodded, up to date version of the game on a really good pc, so I'm pretty sure none of those are the problem.

When I sail to the Isle of mists the save prompt pops up. whether I press yes or no, the game immediately crashes to desktop.

Googling shows that a few people have had this problem, but nobody seems to have a solution. Has anyone suffered this and solved it?

This is not going to help you, I know, but it's just for information sake. The Isle of Mists transition bug was supposedly fixed in patch 1.04. From the patch notes:

"Fixes a crash that could occur on the transition to the Isle of Mists location during the quest of the same title."

Edit: This is a real wild shot in the dark, but if you read the patch notes literally it says "during the quest of the same title", so try selecting another quest as you go to the IoM trigger point -- a long shot, but worth a try.

Another long shot would be to verify the game files or do a reinstall. Maybe it's trying to access some data that it can't find.

I see the bug reported at the official forums and no answer... hope you get it sorted.