To summarize,
Salt water frozen in sealed bottles is inferior to fresh water, as a coolant and a big pain in the'A' if it's spilled.
Sea water added to the esky along with frozen bottles(of any ice) will almost never go below 0C.
The killer slurry is made with sea water with added salt, mixed when it's hot, and then adding a large mass of smashed ice because the freezing process has to start again , as I explained earlier, the dynamic equilibrium has to be tipped in favour of freezing rather than melting. Not enough ice, the brine will never go below freezing and the salt water is diluted.
Freezing salt water is a waste of time.