Saltwater freezes at around -22*C, but in reality the coldest water is found under the Antarctic ice with the temperature of -3*C. I think this should be fixed, Powder Toy aims to be quite realistic after all.
Uhmm no I doubt this would break saves cause the freezing point is not used anywere in saves as far as I know. But I don't think it needs to be saves cause it doesn't matter anyway cause the aim isn't to be realistic anyway like you propose it is, not mean't as offence
The lowest freezing point obtainable for NaCl brine is -21.1 °C (-6.0 °F) at 23.3wt% NaCl.