It sounds like a combination of the strategies suggested above will do the trick Cam's plastic washers, SD's duct tape and the lanyard for the wrench. The button thread with the cork attached sounds like a neat trick too, I'll let you know how it works out. Thanks for all the feedback.
Everybody is making this waaaay more complicated than it needs to be. Here is this professional zinc installer's method:
1.- Disassemble the zinc and lay the parts on the dock. Assuming there are no screw "keepers" (the little red washers found on Martyr brand zincs from West Marine), leave one screw in the zinc half, put the other screw by on the dock. Have a hammer and screwdriver or allen wrench on the dock as well.
2.- Get in the water. Wear a wetsuit glove only on the hand you will not use to turn the screwdriver or wrench.
3.- Tuck the loose screw under the wrist strap on wetsuit glove. Slip the hammer under your weightbelt. Put the screwdriver or wrench lanyard over your bare wrist.
4.- Grab the two zinc halves being careful of the screw you left in one.
5.- Swim to the prop shaft. Place the zinc half with the screw still in it on top of the shaft. Place the other half under the first zinc half, making sure the screw fits into the receiving hole properly.
6.- Holding both halves together with your gloved hand, start the screw with the screwdriver or allen wrench. Once the screw is started, the zinc will not come off the shaft.
7.- Rotate the shaft and place the loose screw that is tucked into your wrist strap into the empty hole on the zinc. Tighten it up snug. Rotate the shaft and tighten the other screw.
8.- With the hammer, set the two halves with 5 or 6 solid blows to each half. Put the hammer back in your belt and retighten both screws. You're done!