Tips on Keeping Ice
We often do trips 3 to 4 days up the Barrier reef and ice lasts that long in the 260 litre. It all depends on how hot it is and how many fish go in it, also the type of ice, etc. I have had up to 7 days camping in 35 degree Celcius days. But this was in a smaller box and I used a combo of block ice and dry ice and drained the water off and don't let the dry ice sit in water. I also froze my meat prior so it was also keeping things cold.
Here's some tips to make it last longer.
Use block ice for longer lasting, but cube or crushed ice cools stuff quicker, but doesn't last as long.
Chuck 2 to 3kg of table salt in with your ice in the ice box. This also helps the ice last. Just chuck a layer of ice in. then chuck handfuls of salt all over it, then another layer, etc. This also prevents freshwater from the melted ice going on your fish. I think this can causes them to go off and it also affects the eating quality and texture fo the fillets.
If you drain the water out of the ice box, the ice will last longer.
But if you are doing short trips, make an ice slurry with sea water and ice. This will cool the fish down the quickest of all. But doesn't last too long. If you have another small box, make an ice slurry and chuck your fish in there first to cool them, then pack them in the big box to store for a few days.