Also bear in mind that Costco is not the kind of place you can dash in and grab a little something for a snack (except at their outdoor prepared food place). There are usually long checkout lines - not optimal for a vacation.

You do not need a membership card to buy their outdoor food, which is a real bargain.

I'd only do this if you are near one -driving out of the way wouldn't be worth it -
But, if you plan on going to Hanauma Bay for snorkeling (which you should) you will be near the Hawaii Kai Costco.