St. Nikodemos of the Holy Mountain, in his interpretation of the 64th canon of the Holy Apostles, says the following:
Fasting is one thing, and leaving off fasting is another thing, and abolishing fasting is still another thing. Thus, fasting, properly speaking, is complete abstinence from food of all kinds, or even when one eats but once a day, about the ninth hour, dry food, or, more explicitly speaking, plain bread and water alone. Leaving off fasting is when one eats before the ninth hour, even though it be merely figs, or merely currants or raisins, or anything else of this kind; or if, besides bread and water, he should eat also some kinds of frugal and cheap comestibles, such as, for instance, legumes, wine, olive oil, or shellfish. Abolishing fasting, on the other hand, is when one eats of all foodstuff’s, including meat, say, and fish, and milk, and cheese, and the rest.
At the link below, Met Kallistos (Ware) describes the common Lenten Fast kept by local Orthodox churches throughout the world:http://aocm-portmatilda.org/fasting.html
St. Gregory Palamas, in one of his Lenten homilies, speaks of the fast of one meal a day, taken in the evening, as being standard for the laity of his day, and that this rule was easy enough that all should be able to follow this practice unless prohibited by poor health. If someone claimed to not be able to keep the fast due to poor health, however, he insisted that such a person be sent to a pious physician to verify this claim and would not simply take a person’s word for it. In principle, the fast of one meal per day, taken after the ninth hour (3pm), should be followed by all Orthodox. In practice, most probably only abstain from certain foods on fast days (meat and dairy) and probably do not limit quantity. If “most” or “many” don’t do what is proper, however, that is of little consequence before God. We will all have to answer for our own obedience or lack thereof, and what the majority does or does not do will be of little help at the Judgment.
In the Orthodox Church, fasting is inseparable from prayer and participation in the Holy Mysteries. When and how to fast correctly in the Orthodox Church is extremely difficult to memorize, since some fast periods allow fish and/or wine and oil on certain days, some days are strict fast days when no food should be taken, some weeks there is no fasting, etc. For this reason, the best guide for fasting is the liturgical calendar. If one is not fasting liturgically, following the liturgical life of the Church and participating in its mysteries, such a fast can hardly be called Orthodox or Christian.
Regarding water, this is rarely restricted even in the most ascetical monasteries and sketes.
Concerning the role of the spiritual father, fasting must be linked with obedience or else it can lead to pride and demonic delusion. As the Fathers remind us constantly, the demons fast perfectly and yet this does not make them holy. Fasting under obedience links fasting with humility and as such may be pleasing to God. Anyone can abstain from food, and such abstinence does not necessarily profit the soul if it is not linked with humility, obedience, the Mysteries of the Church, prayer, and the giving of alms. If a person is weak, a spiritual father may allow a person to relax their fast in order that they not fall into despair, with the hope that the person will grow in strength over time. With those who think they are strong, a spiritual father may relax the fast if a person seems to be becoming proud of “his accomplishment”. If a person is keeping the eating regulations perfectly, and yet is critical of others, judgmental, or indulging in carnal sins, then something is terribly wrong. A spiritual father is essential for helping a person apply the tools of the Church properly for the healing of the soul.