I had gotten two sister kittens when I had to put my beloved Athena to sleep. They were COVERED in fleas. You could actually see them crawling all over their face. I gave them a bath in Dawn dishwashing liquid. A vet tech said that is what they use. The water was RED
Anyway, that didn't get all the fleas, eggs, etc, so I used a flea comb several times a day. I kept a glass full of Dawn dishwashing water nearby, and dipped the comb in the glass every time I combed out a flea. They don't have fleas anymore
Please, please, please do not put anything in your cat's water! Cats are very picky about their water and any "off" flavor will cause them to simply not drink enough, which could lead to bladder and kidney problems as well as dehydration. You can try putting it in canned food, but most cats are smarter than that and will refuse to eat the food if there's something they don't like in it.