In a comment I said:
Yeah, I was looking for this (about "you just wait and see!"); I hear it a lot while watching cartoons
with the kids!
I wasn't confident of (on?) which one sounds natural, until checking with Google ngram (watching cartoon,watching cartoons,cartoon,cartoons). Am I right in choosing "Watching Cartoons" which shows up 20 times as much compared to "watching cartoon"?
By the way, in the context above, is it confident of or on. With Google ngram, on is barely noticeable, and of is almost all there is.
Note: You may want to take out cartoon and cartoons for the other curves to be clearly visible.
Best Answer
I think your usage, "I hear it a lot while watching cartoons ...", is correct. The word cartoon is a countable noun. A singular countable noun will need an article (i.e. a cartoon or the cartoon) or a determiner (e.g. that cartoon). Thus, we can say watching a cartoon or watching cartoons, but not *watching cartoon.
However, your observation made me wonder if "watch cartoon" had become a phrase (as "declare war" did) already.
So I looked into some of the examples returned by Google Ngram, and found that all the "watching cartoon" results (as far as I reviewed) are not a complete verb-ing+noun phrase. For example, All your little friends were watching cartoon shows ..., ... watching cartoon turtles on TV doing ..., ... that children watching cartoon shows for a four-hour period .... In all results, cartoon was used as an adjective.
With that, I think I can conclude that saying *watching cartoon is incorrect.