Back in the old days, we used to call these "swamp coolers" - sounds like something from Florida, no?

The fancy chemical engineering name for it is "evaporative cooling": the cooling effect comes from the evaporation of the water in the wet cloth. So the more evaporation you get, the more your fillets get cooled. It won't work as well when it's really humid outside, and it will work better when there's lots of air flow across the wet cloth. A cloth that has lots of surface area (like burlap) will work better than something with a tight weave like cotton or nylon.

db's bleach idea is a good one: bleach is a *very* powerful bactericide, so you only need a drop or two in a couple of litres of water to kill all the bacteria.

But you don't want to push the swamp cooling thing too far. The restaurant business has something they call the "four-hour rule". According to this rule, you don't want to leave anything longer than four hours at room temperature before it becomes a health and safety issue. I don't know how conservative that rule is, and how much extra time you get from in-boat cooling is up to you. Just sayin' there's limits!