Great question. Since we evolved consuming “fresh”, non-salt, water our physiology revolves around certain set points for normal neurological, renal, and basic cellular function. Consuming salt water changes the osmolality of the blood, which then causes shifts of fluid to try and balance the change of osmolality which negatively effects neurological function since we evolved to function within a strict range of osmolality, sodium level, etc. The body manages this from the pituitary/ adrenal / and renal perspective to maintain neurological function. If you could create a situation where the normal isotonic function is reset to a more hypertonic environment then that would be the start. I’m only familiar with human physiology and pathology but perhaps someone who is familiar with fish physiology could comment on how fish stay “hydrated”.