Is your own press conference going ahead for tomorow?
As for the dating, that will all come out in "the wash" anyway. If the preliminary guess on the part of the NIO turns out to be in error, it will be demonstrably so. With so many organisms encrusting the ruins, carbon dating techniques should be able to easily establish a minimum age for the site (which I strongly suggest will be much older than 500 AD!).
You deserve credit for this find. I am confident you will receive credit for this find.
Well for one thing, the story of a lone researcher arriving at an independent hypothesis concerning something widely thought to be non-existent and subsequently proving its existence is just a heck of a lot more romantic than some archeological institute stumbling across a few ruins on the sea floor. :-) The press loves human interest - and your story is just rife with that stuff! :-)
Christopher F. Ash