Try disabling shadows in the graphics options. This is a known issue (except for trying to save Swan), predominantly on MacBook Air systems, which should be fixed in the next update, currently being finalised for testing. A handful of people have reported this issue recently (the same crash has also been triggered in a couple of cases with a late game act transition, or when trying to join a multiplayer game or create a lobby). Incidentally, the first to report a crash when loading saves reported that if they started a new game and skipped through the ship section to get to Fort Joy, they could save and load any Fort Joy saves, including those from previous sessions, until they quit the game again. Another said that after loading, shadows could be enabled again (though that wouldn't be practical if you frequently reload to try different dialogue options or quest solutions, etc).
