Was he fully burried? maybe his airway was slightly blocked and the water washed it out, he also could've been panicked by something. Did you move something fast near him that may have frightened him? I have witnessed my toads breath heavily or "abnormally" and then be fine when they get in the water. I do believe that water soothes them and calms them. I'm sure it's fine Don't be worried. Let me know if he does that again