Depends on the temps. If it's cool (low 40's) you can keep it raw. If it's in the low 50's you could keep it cooked (or smoked) and salted.

Best bet is to fish in the morning. It's often a good time to fish.

Thing is the dangerous pathogens thrive on protein and moisture. If it wasn't cool it's not a risk I would take though it is more than likely that no harm will come of it.  One possibility on a cool night is salt and a little lemon juice.

Best just to rise early and wet a line.

I usually go paddling in Mid-May, where temps can go from freezing to hot, all in the same day.
I troll as I paddle and if the weather is warm, this is what I do to keep fish cool and edible.
I did it in 2006, and paddled all day in 80 degree temps and the fish was fine for supper that night.
Fillet/gut fish and put in appropriate size zip lock bag. Go to bank and get a couple of the canvas money bags they give customers for change.
I soak bag and secured shut with a Bungee Dealee Bob and put under my seat out of the sun.
About every half-hour, I re- soak the bag.
You would do the same thing overnight, except you wouldn't have to re-soak bag, just soak it good, and hang with food pack or separate,  the bag needs to be free to evaporate to keep the fish cool.
Unless it was during the hottest part of the paddling season, I think this would work just fine.
Others on here put fish in watertight containers and anchor offshore in cold water.
I think in hotter weather, that would be the ticket, but if the night temps are say 60 degrees or less, I think you'd be okay?
Of course, being fish there's no guarantee's and you should do what you feel most comfortable with

To add to Wally's comments.  On the trip where we first saw this idea, we all fished in the evening, filleted the fish, put them in the cooler with the first aid ice packs and ate them the next night.  they were still cool on some hi 80's days.  

Thank everyone for the ideas and advice. I'll be up in the Q end of August so I have a mix of fairly warm days and cooler nights, sometimes very cold indeed. I like both the sink method and the chem pack method.

A side note on the sinking, when filtering water to drink I have a 30' hose extension for my purifier. Attached to it is a 2 ounce fishing weight that brings it down. I can get water thats pretty darned cold for adult beverages or a near ice cold gatorade.   It doesn't stay cold for long, but it sure is nice when its really hot out and the route was portage heavy.

Splice a 20' section of parachute cord onto your stringer. It gives the walleyes enough line to swim away from the turtle/otters. Never had them gone in the AM when I do that.