Over the years I've stayed at several campsites with a local camp snapping turtle in residence.  
We had walleyes on a stringer overnight on one trip, woke up to find a smallish northern with one of the walleyes held crosswise in its mouth.  He didn't let go till we nearly pulled him out of the water with the stringer of fish.
I doubt that fish fillets would go bad overnight, especially if salted a bit and sealed in a ziplock.  I wouldn't try keeping them longer than for breakfast tho.
The sinking into cold water method sounds nearly fail-safe.