We have had good luck keeping fillets for a short time by sealing fillets in a ziplock bag, no air, and putting in anchor bag. Secure anchor bag to a light rope. We then tie a rock to the light rope with a ligher cord, that way if the rock get snagged we loose only the rock.  Sink fillets in deep water close to camp.  

We have been very forunate in having easy access to deep water at almost all sites.
We cool beverages the same way.

Dadman