Corsica and Sardiania have stayed rebel in every single game I've played, until I got there (sometimes 50-100 turns in). Same for Rhodes. Really, Milan, Sicily or someone should be grabbing them right at the start but the AI never seems to bother.
I have also never seen the british isles invaded. Even with the AI being super cautious abut sea travel, Denmark or France at least should really have a go at some point.
The main thing I see the AI do with ships is start pointless little wars by blockading your ports for a turn, and then immediately suing for peace. It's my only major gripe with the campaign part of the game, actually.
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