According to the official website of Hengqin Underwater World () in Shantou, China (圣淘沙), they are open on Tuesdays to Sundays from 9:00 AM to 6:00 PM. Therefore, it seems that Wednesdays and Thursdays are not their operating days.
Please note that opening hours may be subject to change due to various reasons such as special events or maintenance works. I recommend verifying the information with Hengqin Underwater World’s official website or contacting them directly before planning your visit.