Sherlock had heard John cry out many times in his life, mostly various forms of warning, frustration, or anger. Pain was not a common occurance, and certainly not from a source that Sherlock could not even see. The murky water made it impossible to see anything past John's shoulders, and there could be anything down there...

There was barely a second thought as he ran right up to the edge, dropping a foot in the water and splashing it around violently to see if he could distract whatever was down there. Hoping, at least, that it was something alive, and not simply a booby-trap.