Clean it on the boat----always!! Cut the throat or pull the gils immediately after administering the wood shampoo, then clean when there's a lull in the action. Keep fish out of the sun---if no cooler or no ice, get a towel or burlap bag, soak it in the sea, and lay it over the fish, then keep dousing water on it every once in a while
5 days is nothing if you're prepared---salt ice is the best of the best, but any coolant is marginal if you don't drain the melt away from the fish.
Salmon---take guts, gills and blood line out, lay on their backs in the bottom of the cooler on a layer of ice, then belly ice them before covering with additional ice.
If I had a halibut ior ling fillets I would lay them on top of the layer of ice/salmon so there is no contact between the bodies.
Pull the drain plug on the cooler then with plug facing the bilge/stern of the boat, put a 2x4 stud under the other end of the cooler
If your cooler doesn't have a drain option, use it for chilling beer or get it off the boat and get a cooler that does have a drain
Once you've taken care of your fish in that manner, 5 days is not considered a long time, though you'd want to supplement the ice after the 3rd or 4th day