Were you there, and did you see it happen? Do you think it was done out of frustration or, as someone said earlier, a misguided attempt to show your dd that putting her face in the water is not a big deal? Had you had previous conversation with the teacher about your dd's progress? How did the teacher explain the action?
Don't think it was the right thing for the teacher to do, but also think calling it "physical assault" is OTT. Think you need to look for another teacher, because this one obviously isn't getting through to your dd.