Water is one of the main components of icy giant planets (IGPs) like Neptune and Uranus. The electronic structure of water under pressure is of fundamental importance to the construction of the interior models of IGPs, as well as to the understanding of the origin of their abnormal magnetic fields. In this work, the bandgap of water is inferred from the reflectivity/absorption data reported in the literature. It is found that, the electronic bandgap of water in the superionic and plasma phases is 4.4±0.2 eV, in contrast to the threshold of 1.25±0.04 eV for free ion generation in the molecular and ionic fluid phases. The bandgap value is consistent with 4-6 eV predicted for the superionic phase by the density functional theory assuming a hybrid Heyd-Scuseria-Ernzerhof functional.