I'm playing a dwarven fighter, and we've just stared the AP--investigated and cleared out the abandoned village, just advanced to level 2. I have the option to take Aquatic Combatant now, but I am not sure if it needs to be a priority already or if I can wait a few levels before we are fighting in or under water.
J
Avoiding any spoilers:
You are better off reacting to the AP rather than trying to metagame it.
The Player's Guide gives you enough information to avoid nerfing yourself later.