Can a priest make water holy by his will alone in necessity, or must it be properly blessed?

The Church has a certain formula the priest has to follow. If you mean just mental gymnastics to make the water holy, the answer is no. But that is never really a problem, because if a priest has the capacity even to think about making water holy, he can surely make the sign of the cross and say the words required to make it holy. And wherever that water goes, it retains its holiness; this is the Catholic concept that an inanimate object can contain and carry God's grace, so that transporting holy water is transporting God's grace. It is an interesting concept, but it is true.