Nice job! Fooled me the first time. I was expecting a goldfish to be honest because of the water element, so I was surprised when it was a random object like that. As far as advice, there are a number of ways to produce a potato that would work better in person, so if you are looking to use this in real life than this is your new best friend:
http://www.murphysmagic.com/Product.aspx?id=44312
But for video your method worked pretty well except for a bit of movement restrictions. The goal is to always look as free and natural as possible - like you could move if you wanted to. Stiffness arouses suspicion. A laymen would not know how you did it, but they may suspect something fishy (which may be the same as figuring it out -according to Darwin Ortiz in Strong Magic). I didn't know why you cut the potato until after, then thinking back I realized you were just proving it to be solid.
If you want to get really technical, it would be good to bring together elements that all have something in common - like dirt, seeds and water- to produce a plant. That's why I was expecting something like a goldfish.
All that said, nice job on it! Clever method, and I think it would fool many people!
