If the fish is left floating in the water, the temperature will rise to or above the temp of the surface water, which is usually the warmest spot.
Not good for preserving the quality of the flesh of the fish, and also a prime medium for bacterial growth.
By suspending the fish out of the water, but keeping it in a dampened bag, the evaporation of the water from the bag will keep the fish cooled to the lowest temp possible, given the circumstances.
Burlap is the preferred wrapping cloth because the individual "strands" that are woven together to make the bag provide for maximum water absorption, while the relatively open weave of the "strands" allow for the greatest air movement, hence evaporation and cooling, through the weave. If possible keep this out of the direct rays of the sun.
Another approach, particularly in a tube, is to line or carry in one of the pockets several of the frozen gel packs that you get for icing up a muscle injury. In this case, I still bleed, if not clean, the fish before wrapping in damp burlap and laying over the gelpacks. The gel packs are easy to clean up, reuseable, and don't leave melt water in the tube pockets. The burlap still help keeps the fish fresh and also makes clean up of the pocket easier. If its a particularly hot day in bright sun, I may also lay a damp burlap over the top of the pocket the fish is in.
Of course, if you have faith, don't keep a fish at 6:00 AM if you are going to be out for a long period of time. Believe more are coming.
Hope this helps.