I'd just like to add to this list, be careful about icebergs. They also contain salt. I know you all think that the salt should be excluded as the water freezes, but apparently it is not totally left behind. And other things can happen:

"The density of pure water at 0°C is 999.9 kg/m^3 and that of pure ice is 916.8 kg/m^3 [that would be an SG of 0.9168]. However the density of sea-ice may be greater than this last figure (if brine is trapped among the ice crystals) or less (if the brine has escaped and gas bubbles are present.) Values from 924 to 857 kg/m^2 [in terms of SG, 0.924 to 0.857]were recorded on the Norwegian [yay Norway] Maud Expedition"

From Descriptive Physical Oceanography by Pickard and Emery, 1990.