How the Town class's water-filled bulges worked

Were the Town class's water-based protective bulges sealed or open to the sea?

They were sealed spaces, but the ship could pump water into and out of them.

The Town class's so-called water-based armour spaces were sealed rather than permanently open to the sea. Their crews could pump water into or out of them as required. Earlier in the lecture, repeated references to water omitted from the displayed treaty displacement expose the limits of a headline tonnage figure. The answer supplies the physical basis for that accounting joke and reminds viewers that standard displacement was not the same as every load carried in service.