It's not a dedicated sushi counter, but part of the counter seating area next to the show kitchen. There are only about handful of counter seats on either side of a divider of sorts.
Your best bet for going as a walk up would be when the restaurant first opens. I wouldn't chance trying it anywhere near fireworks time. You can go to the check in area on the second floor and ask them if there was any space at the show kitchen counter seating. If not, you can ask to sit in the lounge to order sushi, too.
Like I said, the closer you are to fireworks time, the least likely you'll be able to do this. Even an hour before, you'd probably have difficulty.