Cats can eat watermelon because it contains a lot of water, which can quench their thirst and provide them with some hydration. However, it's important to note that cats should not eat the watermelon seeds, as they can affect their digestion and are harmful to them.

In addition, it's best to feed your cat watermelon after they've finished eating, but not immediately afterward. Wait until they've digested their food before feeding them watermelon. This won't interfere with their normal eating or put extra strain on their stomach. Also, be sure to feed them watermelon in moderation; don't give them too much to avoid causing diarrhea.