Actually, flip flops are fine. But otherwise, not beach attire. Every female in our party had on flip flops, with skirts/dresses. Two of the guys had flip flops, including my son who wore shorts / aloha shirt and flip flops. I wore a long skirt and flip flops. No one blinks an eye at flip flops here. Just make them not $5 old navy plastic flip flops.
And enjoy the meal! It's one of my favorites. We had the most amazing sushi there. Literally - melt in your mouth salmon.