Wouldn't a recyled 32-oz plastic soda bottle, with a few wheel-weights glued inside (length-wise, all in the same area) - then wrapped on the outside with 30' of braided nylon twine ~ to which a basic alligator clip is attached, do about the same thing?
The wheel-weight "ballast" would prevent the floating bottle from completely unloading any remaining twine-wraps after the decending structure hits bottom - and one sharp tug would free the alligator-clip's grip.
