Cats are not able to survive on diets excluding meat. They are what you would call an obligatory carnivore meaning they must have meat to survive. When it comes to feeding your cats fruit & veggies, it’s okay in moderation but not as their main source of food.
Watermelon is something my Siamese cats happen to love. I cut up little chunks and give it to them as treats here and there or put a couple small chunks in their food. The seeds however can be toxic to your cats so don’t let them get a hold of those. I don’t think the rind would be good for them either. Buy seedless watermelon and you are good to go!
Just like with any human food , you don’t want to give them too much too often. Not every cat is the same but it could cause some digestive problems.