My swashbuckling warblade might be fighting a dragon underwater. Can I use Acrobatics to avoid AoO while swimming underwater?
Would there be a penalty, if allowed at all?
Acrobatics skill description:
...you can move through a threatened square without provoking an attack of opportunity from an enemy by using Acrobatics. When moving in this way, you move at half speed. You can move at full speed by increasing the DC of the check by 10. You cannot use Acrobatics to move past foes if your speed is reduced due to carrying a medium or heavy load or wearing medium or heavy armor.
Nothing there appears to preclude using it while swimming. Although swimming does reduce speed, the prohibition here is specifically for load or armour. Swimming is "moving" in the plain-English interpretation, and swimming at one-quarter speed is explicitly listed as an example of a Move action.
That said, the speed reduction for Acrobatics on top of the speed reduction for swimming is going to be a significant limitation for most characters.