Your body does not absorb water from this type of bath - it releases water, so the bath is dehydrating. Whenever you are in really "salty" or mineral-y water, your body wants to equalize itself with the water so that they are the same concentration. Of course it can't fully do this, but anyway, if it's salty outside your body tries to transfer water into the bath to make it less salty, something like that. So water retention is not a possibility, it would be dehydration, and the way the minerals react with your tissues, and detox reactions.