The game crashes when I try to go through the underwater trapdoor or enter the palace.
This is a rare bug which seems to occur for a small handful of people. There's no reason why it should be happening, which makes it impossible for me to troubleshoot. Some players have reported that reloading a save, or disabling all other mods, helps.
Fortunately, it's not necessary to go into the underwater trapdoor to get the best ending.