Such a dumb goal for Arsenal to concede & let Watford back into it, perhaps I'm wrong, but rule one of playing out from the back from a goal kick should be never play it into the box. Most teams that try that typically send the CB's to closer to the corner flags & bring 1, maybe even 2 CM's to the edge of the box to avoid the very problem that Arsenal created for themselves.
If it still isn't on, even the likes of Barcelona wouldn't try to force it, they'd play a quick chipped pass over the forwards into one of the holes of midfield.
Really, really daft, Emery should be fuming with that.