The AI is completely incapable of launching invasions over sea. Not only is this massive and literally game breaking as far as the campaign is concerned, but also, the exact same bug was in the original release of Medieval 2. It's like they either didn't do any QA, at all, or they just didn't care about it. Either way, it's bloody unnerving and the AI is retarded enough without this bug. In fact the AI is like one giant bug.
Also, it turns out the battle map no longer reflects the battle's location in the campaign map, so no more using the terrain to gain an advantage over the AI, for example. Not a bug, as such, but the removal of what was a fundamental feature in previous iterations of the game.