We caught some walleye while rehydrating our dinner for the night.  This is what we did...

We filleted the fish, put their fillets into a ziplock bag and sealed it while evacuating as much air from the bag.  That bag was put into a mesh bag along with a rock.  We tied a line to the bag and threw it into the lake (estimated depth=20-25 ft.)  The other end of the line was tied to a nearby bush.

In the morning we pulled out the bag and found the fillets very cold.  It was a yummy breakfast of walleye.

I would expect that as long as the lake has some depth there is a very cold pool of water nearby.