Cats can drink seawater. Unlike humans, cats have kidneys that can filter out salt and use the water content to hydrate their bodies. Scientists believe that a mutation in a key taste receptor has prevented cats from being able to taste sugar. No cat will ever know the joy of a cupcake. Source: Buzzfeed
You’ll be happy to hear that cats can eat watermelon. However, the seeds need to be removed first, as the compounds contained within them can be very harmful. Watermelon seeds, much like other types of seeds contain cyanide, (otherwise known as cyanogenic glycosides, a harmful natural by product that cats can not process.) If the seed is crushed during consumption, traces of cyanide will be ingested by your cat, leading to cyanide based poisoning. Source: CanCats