Costco or Walmart in Maui
 

Where is the better place to stop for bottled water, boogie boards, beach mats, etc

Or would just the ABC stores be fine for these items?

We got my son a boogie board at Costco. And the bottled water too! I recall getting some other beach items at a Kmart, but I think that was on the BI. I'd go to Costco first, then to Wallmart for what C didn't have. ABC is good for things you really need at the beach/ resort areas and forgot to get. :-)

and Costco, Walmart, and Kmart are all very close to each other so there's not a lot of traveling to do to get from one to the other...have a great time.

Kmart is always a good place to get stuff like that on Maui. Costco would be the 2nd choice.
Wal Mart would not be a choice of mine.

Have a great trip.

I would say Walmart. Most items will definitely be cheaper than the ABC store.

Walmart and K-Mart are also great places to buy souvenirs and other Hawaiian stuff.