If the sight, smell of thought of a certain food mi fa venire l'acquolina in bocca then, as a result ho l'acquolina in bocca (my mouth is watering). Both expressions are of widespread use in Italy. I have always found the word acquolina (which literally is is a diminutive of acqua, water) particularly charming: it dances in my mouth like a tasty morsel.
Avere l'acquolina in bocca should not be confused with acqua in bocca!, which is an injunction to keep something a secret. The idea is that if your mouth is full of water, you cannot talk. I taught my American husband to both say and act acqua in bocca! In fact, as a kid, after saying to someone acqua in bocca!, I would inflate my cheeks and purse my lips slightly, simulating a mouth full of water. I am no longer a kid, but I still get to do it with my husband and then we both burst into laughter (scoppiamo a ridere).
[This post was inspired by Valli, who used the adjective droolworthy in a comment on a post and got me thinking of Italian acquolina.]