A hull for deployment as an underwater habitable vessel is described. The hull has at least two hull wall modules connected in series to enclose an interior volume of the underwater habitable vessel. The interior volume is bounded by the hull wall modules. There is also provided a kit of parts for a hull of an underwater habitable vessel. The kit comprises at least two hull wall modules configured to be connected in series to enclose an interior volume of the underwater habitable vessel. The interior volume is bounded by the hull wall modules when connected.