Water scarcity caused by the growing population and pollution in the wake of natural disasters or supply system damage is a severe threat to our society. Thermal and membrane-based technologies have been developed to deliver clean water via seawater desalination and wastewater recycling; however, the requirements of high energy consumption and frequent maintenance of infrastructures have raised a barrier for off-grid regions. Solar-powered distillation has regained attention as a potential solution to water shortage because solar energy dwarfs all other fossil-based energy resources in abundance and environmental benignity. A feasible solar-assisted water purification system not only requires scientific breakthroughs to enable efficient energy capture/conversion and a high rate of clean water production but also needs to be affordable for communities that suffer from economic water scarcity. Hydrogels are an emerging material platform for applications in sustainable water-energy-environmental nexus because gelation chemistry endows them with tunable physicochemical properties and ease of integration with versatile functionalities. This Dissertation focuses on designing hydrogel materials for solar desalination and water purification technologies. First, solar-absorbing nanoparticles are introduced into the hydrogel matrix to efficiently harvest and convert solar energy, powering water evaporation within molecular meshes. The energy utilization and water transport of the systems are investigated based on tuning polymer-water and polymer-particle interactions in hydrogel networks. Next, since evaporation occurs at the hydrogel-air interface, the surface property of hydrogels plays a vital role in the interfacial evaporation process. Taking advantage of intermolecular interactions between polymer chains and solvents, the surface topography of hydrogel evaporators can be tailored in nanoscale to achieve an enhanced water evaporation process. Furthermore, through chemical functionalization, surface wettability can be regulated to boost water evaporation performance. Last, by modifying the functional groups on polymer backbones and absorbers, heavy metal-ion adsorption, antibacterial, and anti-biofouling features are incorporated into hydrogel evaporators. It is anticipated that the studies presented can promote a better understanding of the water evaporation behavior at the hydrogel-air interface and inspire the future design of solar distillation devices/systems with higher clean water production and lower cost, overcoming the dependency on electricity supply to achieve decentralized water purification.