I see the problem with using acrobatics now while swimming. I guess if you were hasted that might allow you to use it.
I think in the situation that came up, we were actually walking along the bottom, so we were moving at half speed, but that is still a very short distance.
you are also only taking that acrobatic penalty for the square you are moving through correct?
I see you can also take a -10 penalty to move at full speed with acrobatics. I guess a check with a -10 penalty is better then no check at all.