Lock 21 is a shore dive in the Long Sault area. There are a couple of reasons that it is not moored by SOS. One is that the guidelines that are already on it are constantly breaking under the strain from the weeds that collect on the lines from the current and a buoy would only stay afloat for a few days unless constantly cleared. Another is that SOS is a charitable volunteer organization which must be careful with what little money it has. Buoys are expensive and volunteers to maintain them are hard to find, therefore shore sites such as lock 21 and prescott docks are not moored. Although it is largely a shore dive, the connestoga was moored because there is an issue with boating traffic from the fishing community over-running the site at high speed and creating a hazard to the divers below.
IMHO if you are diving Prescott and feel the need for a flag, take your own. Instructors should be teaching their students to take and use a flag or float, not teaching them to expect one to be there. I always carry a marker and spool in my backplate so if I need to I can deploy it, otherwise I am unhindered by it.
Kevin