You can do a search for boat recovery
, retrieval boat
, rescue boat
, tow line
or any other related terms.
Radio Control Boat Modeler Magazine did a how-to article for building a grapnel with some dowel rods, rubber tips, and some line. IIRC they used a 3/4" dowel for the main shaft, about 12" long. At one end they drilled three or four holes at an angle - think arrow head. They glued some dowel rod into the holes forming a hook, these dowels were about 6" long. The whole thing was sealed with epoxy and painted a bright color. On the ends of the hook dowels hook end of the 3/4" dowel, they installed some rubber caps. At the other end of the main shaft, a screw eye was installed, and a line was attached with a swivel.