That is your belief, and you are entitled to it. It is my belief they are real, in that they really do exist. This is not religion any more than the Higgs Boson was a religion before it's discovery.
What is really making the experiment impossible is the infinitessimally short amount of time they exist. Have we even detected any virtual particles yet? Let alone virtual photons?
Why is it tricky? If pushed down it will stay at the new depth unless acted on by another force. The force of water pressure would be equal on all sides would it not? Although there is less pressure exerted on it near the surface than there would be at greater depth. But that could be negligible...