There is a single scientific study or report on the topic of cats drinking seawater and whether it is viable or not. There’s lots of anecdotal evidence from commenters on social media saying that cats can drink seawater (salt water) because their kidney function is superior to that of humans and they are, therefore, able to process seawater and remove the salt from it. However, the Pet Poisons Help in the US states that salt is toxic to cats and they should not drink seawater because of its high salt content. Confused? Because there’s only one dubious (?) study on this I would have some reservations about its conclusion (link to study summary – note links to external sites sometimes fail).
The study I’m referring to is titled “Potability of Seawater with Special Reference to the Cat“. The researchers state that when a cat has an adequate amount of food but not enough water, they can sustain themselves by drinking seawater. Or they might eat food mixed with seawater.
Humans can’t drink seawater because their kidneys can’t cope with it which leaves salt in the bloodstream which makes them thirstier which in turn means they drink more saltwater and a negative cycle develops which leads to acute dehydration and potential death.
Human kidneys can only produce urine which is less salty than human blood and salt water contains more than three times the amount of salt normally present in human blood.
Saltwater can never quench thirst. It only makes you thirstier. Certain marine animals such as seabirds and marine mammals have adapted to drinking seawater.
A veterinary handout on salt (NaCl – sodium chloride) poisoning cats states that only a few grams of salt per kilogram of body weight can be hazardous. Even a teaspoonful of salt is potentially dangerous to a cat. It can be found in a variety of sources including seawater (obviously). The oceans are so salty that if you removed all the salt from them and spread it over the land encompassing the entire earth it would stand 500 feet tall.
A high blood sodium concentration can result in death in a cat and the symptoms are numerous including vomiting, diarrhoea, lethargy, thirst and excessive drinking, tremors, weakness, dehydration, stiffness, rapid breathing, convulsions, kidney damage and coma. They make it clear that cat owners should not allow their cat to drink seawater.
It never occurs to cat owners for obvious reasons but it is relevant to answering the question in the title. It is ironic that one of the most prevalent causes of death in domestic cats is kidney failure. The commenters and experts appear to be saying that a cat’s kidneys are superior to those of humans (which I doubt to be honest as there is very little difference in basic anatomy) which is why they can process saltwater and humans can’t but at the same time something we are doing to domestic cats causes their kidneys to fail far more often than is the case with humans. Still confused?
P.S. In another study on mice drinking seawater, they concluded they can: “Two species of mice…lived 1 year on a diet of seawater ad libitum and dry food. High urinary osmolarity [efficiency], high ability to concentrate sodium, and the absence of diarrhea were factors that allowed them to endure this experimental diet.