Kubo had the foresight to carry his shamisen instead of just keeping it on his back, so he was able to keep it out of the mud (and uncrushed) when he landed on his back in a puddle that immediately swallowed up his lower half.

"Um," he said, sitting up, holding the shamisen over his head. At least the air was warm enough that being damp would only be a bit uncomfortable. "I think I'm stuck."

He looked around to see where his parents had landed, and which one was in a better position to pluck him on to dry land.