Water plays a pivotal role in a wide array of physical and chemical processes. For example, in several batteries, photoelectrochemical cells, and bioelectronic devices, solid-water interfaces are present and critically influence the devices' properties. Water also constitutes a major portion of the Earth's crust and mantle, participating in several geological processes under high-pressure, high-temperature conditions, including metasomatism, carbon transport, and continental crust evolution. To understand the electronic characteristics and structural modifications of water molecules at interfaces or in extreme environments, computational modeling at the atomistic scale is an essential tool. In this thesis, we employed first-principles simulations to study semiconductor-water interfaces and water under extreme conditions. We focused on hydrogenated silicon (Si) surfaces interfaced with water, given silicon's widespread use in electronic devices. Furthermore, we investigated water under pressures and conditions relevant to the Earth's interior (11 GPa and 1000 K). In our interfacial studies, we explored the electronic structure of the hydrogenated Si(100) and water interfaces by performing first-principles molecular dynamics simulations in the presence of an electric field. We correlated the computed flat-band potential and tunneling current images at the interface with experimentally measured capacitive and Faradaic currents. Consistent with chronoamperometry measurements, our simulations indicate that the capacitive currents at the interface are voltage-dependent, while the Faradaic currents are weakly dependent on the applied voltage but are related to surface defects. Next, we investigated the dynamic and vibrational properties of water at the electrified interface. We analyzed the H-bond structures and orientation of water molecules, and we related the structural properties of interfacial water molecules to the OH stretching mode in Raman spectra. The calculated spectra reveal a combined effect of the surface and the electric field on the Raman features observed at the interface. The presence of the surface leads to low-coordinated hydrogen bonding configurations and, hence, a blue-shift of the O-H stretching band relative to that of bulk water. The electric field regulates the orientation of interfacial water molecules, resulting in a stable H-bond network that gives rise to specific Raman peaks in the low-frequency region of the spectrum. Our computational studies provided comprehensive insights into the electronic and dynamic properties of Si-based electrochemical or photoelectrochemical devices. In our study of water in extreme conditions, we carried out calculations of photoelectron spectra of water and a simple solution of NaCl under high pressure and high temperature. We combined first-principles and deep-potential molecular dynamics with dielectric-dependent hybrid functionals. We found notable changes in the spectra relative to ambient conditions; in particular, we observed anion energy levels closer to the valence band maximum of the liquid than those observed at ambient conditions, indicating that as pressure and temperature are increased, the defect levels of chloride and hydroxide ions in water may eventually lie below the valence band maximum of water. We also elucidated the electronic states associated with proton transfer events at high pressure by calculating the projected density of states. Our results represent an important first step in predicting the electronic properties of solutions in supercritical conditions.