I was going to counter and say surely they cleaned it up a bit before posting that pic?

But then I googled for other gladii found underwater...

it just doesn't look right to me, after looking at all the other gladii artifacts.

roman gladius artifact underwater (in google image search) -- there's a few hits there.
Oddly, Pinterest seems better for that search ;0

It's just not nearly corroded enough for the duration of submersion it's alleged to have endured.
Barnacles/other organisms might have been cleaned off, but it looks like you could polish it up and sharpen that edge and make it usable. That's not right for real artifact is it?