Sinking ships to block harbors, canals, and docks

How often were ships used to block canals and dry docks in wartime?

Regularly; sinking blockships in narrow approaches has long been a standard method of denying maritime access.

Deliberately sinking a vessel across a narrow channel is described as a standard practice as old as defended harbours. A blockship supplies an immediate physical barrier when defenders lack a gate or another fixed obstruction. The same principle can deny a harbour entrance, canal, or dock approach. The answer treats the practice as routine across naval history rather than an unusual expedient confined to one war.