it would be a lot of tomato soup

i guess in the case of humans we would have evolved for a long time to mainly eat fruit that has a high water content with low salt content possibly even being most of needed water intake. after starting to eat meat perhaps there hasnt been enough evolutionary pressure to be able to regain saltwater consumption ability

if humans would be adapted to that it could work but it would mean without modern technology it would be close to impossible to survive without access to saltwater (most of human habitable land area)

but whales like all sea creatures have high bodry salt content making the osmotic pressure difference small

yep, usually the first organ to fail in old cats, so the superpower seems to come with a drawback. edit: removed inaccurate statements

kidney disease is one of the most common ways cats die of old age so super efficient kidneys dont come without a tradeoff. Cats have evolved to live in very arid enviorments where saltwater is all that is availible so the tradeoff might have been worth it. ability to drink saltwater only would work without kidneys being prematurely overstressed would be likely if animals had higher normal salt content but that would mean they would need a lot higher salt intake making living inland harder.