My understanding is that salt ice is most effective in a slurry, and a slurry is effective at cooling down fish because it comes in complete contact with the fish rather than just the pointy edges of ice cubes or even flakes. If the liquid portion of the slurry is salty it will obtain temperatures below zero Celsius and insulate the ice better than if it were zero Celsius. Also, water takes longer to cool than ice (specific heat density or something...) which may be a benefit over just pure ice. I have no idea how much salt to add but I'd try to keep the slurry at least under 30 ppt (~seawater), and probably way less than that. I was just reading up on this for an upcoming trip but I didn't come across any definitive recipe. Hopefully more people chime in.
Like Matsutake said. You are probably better off to make your own ice and bring it with you even if it means travelling with it. A normal chest freezer will bring ice down to ~-20C, way colder than the -3C stuff you buy at most gas stations. The the larger the chunk the better it will stay frozen, but just remember to either break it apart when you use it to cool fish or at least dump in some seawater to create better medium to transfer the heat.
