The Grand Saga
Aquatic characters must be able to breathe water. Naturally breathing air or being able to do so through virtue of a magic item is something you should try and accomplish by 6th level.
Aquatic characters are divided harshly on one issue. To hate or not to hate the surfacers. While this doesn’t really affect party cohesion, the overall attitudes of the characters may affect where the campaign goes from there.
Because most of the campaign is underwater, you should think carefully about using ranged weapons due to the penalties assessed to ranged combat underwater.