Walking on water was a miracle. But much more dramatic miracles were possible in the previous ages. It is your faith that makes miracles possible. When God is more fully accepted in human society, people are basically rewriting the algorithms of creation itself. Shrawan Kumar's parents cursed King Dasharath saying may you also experience the tragedy of losing your son, and it happened. Lord Rama Himself was that son.
If you had faith the size of a mustard seed, and you asked the mountain to move, it would move. It has happened before. Hanuman, the famous devotee of Lord Rama, the Holy Father Lord Vishnu in human incarnation who was on earth about 7,000 years ago during the Treta Yuga, moved mountains. Lord Krishna held a mountain on one finger.
Walking on water is not a revision to creation. The decision was made at the moment of creation that you will not be able to walk on water. It was a perfect decision, no revision necessary.
Walking on water is not saying, now on boats and bridges will no longer be necessary, what a nuisance!
Miracles establish the identity of God. Prophecies fulfilled also establish the identity of God. You get to see for yourself that God is indeed omniscient, omnipresent, omnipotent.
Knowing that there is a God is a stretch for a lot of people. But that knowledge is not enough. The Devil has that knowledge plenty. You also have to choose to do God's will as the best of all possible options. What happens when you do God's will completely and collectively? You end up with the best possible human society. Obedience is liberation. Rebellion is slavery.
Water, by the way, is my favorite substance on earth. It is not gold or silver.