It is just like humans too when we are sick. Injections are the best when it comes to effective treatment, but those that cannot stand it can go for drugs.
They enjoy being in water? That is strange, I wonder the kinds of cats you have. Cats generally do not like being in water, and it takes grace to be able to bathe them.
Although there are times when we cannot really be sure, because these cats are predators. They can eat mice too provided they catch one in the house. This can affect them if the mice have infections.
There was a time I bought a jar of cookies and I said I would eat it for 4 days. I finished it all the very first day. So I know how it feels like to have easy access to food that I was supposed to spread over some days.
This is not different from self medication when we are sick and it is something that the doctors always warn against. I don’t even try self medication nor will I do it with my pets.