Drawing a boundary across the sea cannot close it to shipping. Enforcing that boundary requires forces already in place and a willingness to sink ships that try to pass. Even then, the ocean is too large to search everywhere. In peacetime, maintaining ships in different regions provides the presence needed to watch events and respond. This is why the Royal Navy distributed cruisers around the world.
Chapter · 2:13-3:14
Sea control depends on ships already present
An ocean cannot be sealed like a land frontier, so surveillance and enforcement depend on forces distributed across its immense area.