To get this scene I think you literally need to go to Mystra first before you long rest for the boat scene. And then it's easier to convince him against it in the Tabernacle. If you go to the boat scene first, you have to be completely against the idea.
This was apparently available from the original version! There may be some unknown steps inbetween, but what I tried (that worked) is the following sequence: 1) Upon getting the Karsus book in the vaults of Sorcerous Sundries, give it to him only at the third chance/deny him twice. 2) Walk out of the tower and Elminster should be there. Interact with him as your PC and say nothing when the option is available. Or at least try dissuading Gale if there's no option to keep quiet. 3) Head to the Mystra idol at the Tabernacle and let it play out. If Gale challenges her then he's still half a mind in seizing the Crown and this sequence doesn't happen. Hope that helps!
Woooow, I've never seen this version of the scene before. No matter how much I tried to convince him to forget the crown and become a god, he always stubbornly sticks to this idea during the boat scene. Is this the new option in patch 6?
