Mac adding strength of spirit to general excellence (10)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

PROOF(a scaled measure of the strength of distilled alcoholic spirits) plus ( adding … to ) WATER(excellence/quality, as in the degree of brilliance in a diamond, and in the phrase “of the first water” – I presume “general” is included for the surface reading).