your catch should be fine for a typical day of fishing as long as the best is done to keep it that way. Putting it in the water is the one of the less desirable things to do. It will actually raise the temperature of your catch more than air will because heat energy will transfer more readily from the water to the flesh than from air. Submerging it in water allows the meat to absorb water which makes it swell and get soft. Keep fish moist. Evaporation uses heat energy and will remove it from your catch. Keep it in the shade, in cool moist grass or in a burlap sack. Don't leave it in the sun. I remember when I was a kid and went bar fishing in the summer with my father. We put our catch in a plastic bag and buried it in the sand, dug it up when we left and it was cool and fresh. There are other ways of putting your fish in a make shift cache like that.
Clean the fish as soon as possible particularly if it has been actively feeding before it was killed.