My arguement of proving Jesus divinity still holds. If you HAD lived in the time of Jesus, and did see Him like those who saw Him did, you would have had no proof that He was God. None, except His testimony, or the testimony of those who had seen Him. Your argument about the past does not hold because if Jesus was here now, you still would not be able to prove His divinity.
As for the water argument...
When I see a chair, even if I had never seen that particular chair in my life, I would be able to recognize it as such. Why? Because all chairs have certain properties that I can recognize, and yet, the chair I am viewing can be like no other. While it's form is different from all other chairs, it has the "substance" of a chair. It has that unique "stuff" that qualifies it as a chair. And even if I paint it a different color, or make it out of a different material, it is still a chair, because it still has the "substance" of what makes a chair.
So what am I saying? The "chairness" of a chair is a mental construct. I have a notion of what a "chair" is, and thus I can force that mental contruct onto a real object. I have an idea of what a chair is, and I can recognize an object that qualifies as a chair if I encounter one.
So, once I have this chair, I can change it up, and it is still a chair, because that it what it is. I have deemed it a chair, and it will always be a chair. However, if what looks like a chair, I defined as a "desk," it would cease to be a chair and instead be a desk. Why? Because this is all arbitrary: the substance and the form are separate entities.
I can make a desk that looks like a chair, and it will be a desk because I have defined it as such. It has the form of a chair, but its substance is now that of a desk.
Thus, in the Eucharist, if God wishes to make the "substance" of the material on the alter "the Body and Blood of Christ," this can occur without a neccessary change in form, and not just in mentality, because God's Word does not return to Him empty: it achieves what it sets out to accomplish. The substance, or the WHAT of the "object" is different, even though its form has not changed.
NOW, is this Catholic teaching? I doubt it. What I just wrote is overly complex and philisophical, and perhaps has holes in it. This is my way of thinking through that which cannot really be thought through, because it is an article of faith, not science.
The Catholic belief in "transubstantiation" is our attempt at defining an article of faith, which IS true, even if we do not understand it. In the Roman Rite, we define it as such. In the Eastern Rites, they don't use the same language, but hold to the same belief. Thus, to attack our theology, or understanding of faith, does not do away with the Truth of what occurs. It simply means our human language is limited when attempting to understand the omnipotence of God.
God bless,
Grant