I suppose the obvious thing is to give a vet a ring, and ask them, if it's just irritating to the ferret and nowt too serious.
But if you do add something to the water, DON'T add TCP. TCP contains phenols, and ferrets react badly to phenols, they are poisonous to ferrets.
You can tell a substance has phenols in it, as they turn cloudy when you add them to water.