Is it essential for a priest to add a drop of water to the Chalice at the Offertory?
Well, it all depends on what one means by 'essential'. It is not essential to validity. If a priest fails to do this, bread and wine are still transsubstantiated into the Lord's Body and Blood; the Holy Sacrifice is still validly offered. In general terms, it is very difficult for a priest to render a sacrament invalid.
Sometimes anxious Catholics wonder whether a Mass is invalid if the celebrant (for example) does not actually believe in the Mass. "How can he intend to offer Mass if he does not believe in the Mass?". It sounds like plain common sense, but in fact it is contrary to the teaching of the Church. This is why an atheist or a Moslem can, in emergency, baptise a weak newly born baby, even though he/she does not believe in Christianity, still less, in Baptism. The basic intention to 'do the thing that Christians do', is sufficient. (And, of course, the use of water and the basic words.)
The Holy Office once had to decide on the validity of Baptisms performed by a nut-case priest who believed that by baptising a baby he was consigning it to the Devil!! The answer was that his Baptisms were valid. However misguided his views, as long as he was intending to do the thing called Baptism, it was valid. On another occasion, it came to light that Methodist missionaries in Oceania were actually saying in the course of the Baptism Service that it was merely a symbol and did not confer Regeneration. The Holy Office declared that even this public declaration of blasphemous heresy did not invalidate the baptisms.
The basic reason for this is that the Sacraments are the Sacraments of the Lord, and He is faithful to His promises.
The only way a priest could invalidate the Mass would be to use substances other than wheaten bread and wine of the grape; or to omit the crucial words of Consecration; or to form an intention deliberately not to consecrate ... out of hatred, perhaps, for the congregation or for the Lord ....
The apprehension among some Traddies that Novus Ordo Masses may often be invalid, because of a defect in the priest's 'Intention', is WRONG, WRONG, WRONG. S Robert Bellarmine explained carefully that if a validly ordained priest became a Calvinist, and, believing that the Church of Geneva was Christ's True Church, intended to celebrate the Lord's Supper as the Calvinists had received it, his (dreadful and sacrilegious) service would still be a valid Mass.
Heresy, or even complete unbelief, on the part of a Minister does NOT invalidate his sacraments. Not ever.
I shall not enable comments which contradict this explanation. The matter needs to be understood. It is not up for discussion. It is what the Church teaches, and has taught for centuries, and has embedded in her praxis.
So omitting the drop of water in the Chalice would not invalidate a Mass. But it would be highly unbecoming and unpriestly. The ancients generally mixed water with wine; our Lord will undoubtedly have done this; and Bishops and Priests have faithfully continued to do this ever since. For nearly two millennia!! But it is not required for validity.
A priest who fails to do this is probably just being absent-minded. This is something that can happen to any of us! Especially as we plumb the ever-more-profound depths of senility!
It might be kind, very respectfully and deferentially, after giving him a whiskey or three, to tell such a priest that his omission is something that some members of his congregation find distracting.