I throw my fish on the rear deck and cover them with wet burlap bags. I also dip the fish back in the water occasionally.
I NEVER put them inside my kayak's front hatch (Malibu Stealth) because it seems to get warm in there.
Another reason that I like my fish on the back instead of in the front hatch is that I have better control of my kayak when coming back in through the surf. I've come close to pearling with nothing on the front and don't want 25+ pounds up there pushing down the nose.