The serial in question is indeed THE SHIELDING SHADOW (1916), and the unfortunate fellow being dragged into the water is leading man Ralph Kellard, who also starred opposite Pearl White in PEARL OF THE ARMY. According to some sources, he was the father of Robert Kellard, juvenile lead of DRUMS OF FU MANCHU and star of PERILS OF THE ROYAL MOUNTED and TEX GRANGER. But I haven't confirmed that.