I'm not sure exactly what this is. It's clearly made by human hands and it might be a miniature fish pond or an artificial rock pool. It wasn't there last time I visited this spot in Puako on the leeward side of the Big Island of Hawai`i. I used to visit this place a lot as it's a great place for turtle watching but haven't had the time in the last couple of years.
Feel free to click on the pictures to see larger versions.