Gig 601XL Builder writes:

I don't know about the low tide part but it is rocks and it says so on
the chart. Bottom right corner of the legend where it shows a lake,
damn, pier and rocks.

I found it. I didn't expect the symbol to be combined with others in the
legend. I also found it in the chart guide, although it's so tiny (at least
on the screen) that it isn't recognizable as the same symbol that appears on
the chart.