How low-temperature water develops the formation of strong hydrogen bonds with some network structure is still open to a question. Heat capacities of the water confined within silica MCM-41 nanopores with different diameters in the range 1.7-4.2 nm were measured by adiabatic calorimetry. They revealed a hump with its maximum at 233 and 240 K for ordinary and heavy water, respectively. The maximum temperatures were essentially independent of the pore diameter, whereas the maximum values increased only in proportion to the fraction of the internal water molecules within the pores. It was concluded that the manner in which the hydrogen-bond formation progresses in bulk water is essentially the same as that in nanopore water and that strong hydrogen bonds are formed on cooling by arranging the neighboring water molecules at tetrahedral positions but keeping their network structure irregular to make striking contrast with ice structure.