1) I have never stated that the volumes are additive. To do so would be to promote a falsehood.
2) I have stated (repeatedly) that the density of salt water is greater than that of fresh water.
3) That if you mix some salt water with some fresh water the resultant liquid will have a density intermediate between the two.
4) The the volume of the mixture will be equal to the mass of the mixture divided by its density.
Which of these statements do you disagree with?