The water/wine quote appears in a letter SLC wrote to W. D. Howells on
2/15/1887 :

"...high and fine literature is wine, and mine is only water; but everybody
likes water."

Some sources also give an earlier quote which appeared in Twain's notebook
of 1885:
"My works are like water. The works of the great masters are like wine.  But
everyone drinks water."

Barb