Salt lowers the freezing point of water, so when tuna fishing
we keep a saltwater slush to quickly cool the tuna. The slush is typically about 17 degrees, even though the ice itself is just a few degrees under freezing. Lots of people are surprised by that result, especially since no chemical reaction is involved and neither is any expansion of compressed gas (the more common ways to provide cooling
below ambient temperatures). But the basic reason is because it takes lots of energy (and taking energy means cooling) to change the state of water from frozen to liquid. But that quantum of energy is the same for freshwater ice as for saltwater ice. Nothing is gained by starting with saltwater ice as compared to freshwater ice of the same temp.