Dredge cutter chain came out of track. A chain come-a-long was secured to the cutter chain to reposition the links. The come-a-long chain slacked off, engineer reached under the water line to secure chain, the come-a-long chain slapped. 2.1 burst laceration to web between pinky and ring fingers. Prescrip. meds, stitches and drain to left hand. Mod. duty, no left hand work.