See US Chart 1.
Section N - Areas and Lmits
2.2 Prohibited area.
They are telling you that all the area so marked within the dashed red lines is a "Prohibited Area".
If you click on the icon, you will see that there are several sets of dashed rectangles. I'm fairly sure, based on the "Prohibited Area" annotations within the area containing that symbbol that the relevant entry is the "Restricted area - Entry prohibited", (just below the "Bottlened dolphin...." entry ) and it's not marking the wider "no discharge area".