Not Enough Water?

Faceted beaker
Barnwell, England
Late 1st century A.D.
Capacity, 0.31 liters

Conical Jug
Radnage, England
Mid 1st century A.D.
Capacity, 1.1 liters

If the etiquette of mixing water with wine was intended to encourage sobriety, it certainly didn't always work:

"Boy, mix me trientes half and half, such as [the slave] Pythogoras used to give to [the emperor] Nero. Mix them, Dindymus, and not so long between them. I can do nothing sober, but when I drink, fifteen poets will come to my aid...." (Martial, Epigrams XI.6)

and

"Come, boy, you who serve out the old Falernian, fill up stronger cups for me, as the law of Postumia, mistress of the revels, ordains; Postumia more tipsy [ebrioso] than the tipsy grape. But water, be gone, away with you....water, the destruction of wine, take up your abode with scrupulous folk." (Catullus, Poems XXVII)