mr. seas, dry ice works very well but you need to handle it very carefully. Don't touch it or let it touch the fish. Wrap the dry ice in newspaper and keep the fish surounded by it and packed tight in the cooler. I don't know what kind or size cooler you have but you mentioned that it would not keep for the 3 or 4 day trip. Dry ice may not either then. Dry ice can be a little pricy, so consider how much you will need in relation to the amount of fish you have and how much you will need to surround the fish and pack it tightly. You may be better off getting a new cooler. Coleman or Igloo, I forget which one, has a "deep freeze" cooler that holds the cold longer then the "regular cooler". They sell them at Sports Authority. I think a "regular" Igloo cooler, a marine one in white, could work too. Really depends on how much room for ice you have in relation to the fish. With a regular Igloo, just check & drain the cooler daily and refresh with cold ice. Remember, cold ice and pack tightly. I probably have every size Igloo cooler made, they are very good and can hold for a good long time.