What is needed is a product that will either sheet or bead water on plastic. What Pete mentions in his comments on disolving paraffin in diesel is doing exactly this, putting a wax coating on plastic. Choosing the proper parafin is important here as the lower melting waxes will not work as well and may actually increase the chances dirt will stick to your decoys - this is not a good thing.
What I do is wax the decoys. One can choose from a variety of waxes but the one I use is the 3M Perfect It Show Car Paste Wax. Couple of reasons for this choice - I get it free, I know what is in this product and the chemicals are not hyped at all. I developed it to smell pretty when applying so my wife didn't complain about how I smelled like solvents everytime I came home from work. I tried the fragrance "beer" for a hoot and did that one really stink up the lab. Sprayed some of this fragrance around the lab after an all nighter party the college students who worked in our lab attended. Should have seen the green faces when they sat in their seats. Stale beer smell and really strong. Sent a few of them to the bathroom a couple of times. Too funny.....
http://www.properautocare.com/pershowcarpa.html
What is nice about this wax is that it contains both a silicone and a fluoropolymer so the coating will repel both dirt and water. It also has a couple of othe goodies that will really bead the water and with any rocking motion of the decoy, sheets that water off the decoys.
Other options that I have tried with varying degrees of success:
1. Cross Country Ski wax. Mostly paraffin with varying length of polymer chains. Good water beaders, not the easiest to apply and wipe clean
2. Rain-X (and Prestones version of Rain-X) windshield coating. Works OK depending upon the material used to make the decoy. This coating is formulated to stick to glass, or a component of glass. If it sticks to the decoy, this makes a good coating.
3. Mr. Clean's car washing detergent. This contains a unique set of surfactants that actually will sheet water which is actually a better choice than beading water. This works well but needs to be applied frequently. Also doesn't work very well on a surface that is less than perfectly cleaned.
The best coating, if I could find out what it is, is used on Lexus automobiles on their side windows. Whatever this is, it really sheets that water and would be perfect for coating decoys.
Oh yeah, I don't know how any of this would work in a salt water environment.
Mark W