sorry this isn't an answer, but i'd be interested in similar myself.
Saw it in use a couple of years ago, approaching a beach we threw out the stern anchor on the bungee, ran in to the beach and unloaded the main anchor, tied off with enough slack then let the bungee pull the boat back out in to deeper water. It saved anyone having to adjust the anchor for tidal movement (over a couple of hours only) and also anyone having to swim out to recover the boat after lunch - just pulled it in against the elastic when wanted to leave.
Any source of 'used' bungee cord? I think the stuff we saw in use was actual bungee jumping cord, elastic inside a protective nylon sheath.
Anyone able to suggest suppliers of this?