Various warning markers, like the one above off sunken ledge in Quincy Bay, are all over Boston Harbor. The marker above is a red light beacon sitting on top of Sunken ledge. Other places one might find a similar beacon in Boston Harbor include the quarantine rocks, and the rocks at the western end of the Brewster island spit.
Below is a non-lateral navigational aid, known as a diamond day board. These markers are placed to make mariners aware of their location and position. They are typically placed on a landmark or at the entrance to a channel.
Below is a red day beacon, denoting a large and obvious rock outcrop. A day beacon does not have a light on top, like the larger beacon in the top picture.
Nix’s Mate, possibly the most ominous and dangerous place in the Boston Harbor, is marked with a very unique warning marker- a red and black cone. The picture of low tide below is a reminder of how hazardous this place is to navigate: