mouthwatering might have come by conditioning ,because the response to the stimulus becomes associated with the other causing "mouthwatering".

The waiter placed a mouthwatering meal in front of a group of ravenous customers, and immediately regretted it.

mouth watering

i think James said in the book twilight, that Alice had very mouthwatering blood.

Yes, "mouthwatering" is an adjective used to describe food that looks or smells delicious and makes one's mouth water in anticipation of tasting it.