May I know why there is no such word in many dictionaries ?

It is supposed to be a water mark left after something is being washed.
Isn't it two words: Water spot?

I finally got it from the above link. Yeah, it is supposed to be in two words. But I saw it in one in an advertisement. And strange to me that it uses it as a verb too.


I fail to locate the ads anymore. But I've copied down the sentences and pasted in on the above link.