To really seal them well he's going to have to repaint them. I assume the are black cork.
I would clean them up and hit them with some sand paper. Then do at least one layer of titebond II or III waterproof woodglue mixed with cork dust or saw dust. It will fill some of the voids and make a good hard waterproof shell on the decoy. Sand after it dries and do another coat.
Now you can seal with a number of things. Val oil, spar varnish, GAC 700, Jansens cork sealer. It would depend on what kind of paint you are going to use.
I like Goldens heavy body matte acrylics. I would use a few layers of Gac 700 to seal the coating. Then paint away.
The rougher the finish you leave the more pain it will take.
Good luck with your project.
-D