Durability enhancement in Ultra-High-Performance Concrete (UHPC) is of growing interest due to its application in aggressive environments such as saline and geothermal conditions. This study investigates the self-healing performance of pre-cracked UHPC under sustained tensile stress when exposed to tap water, saltwater, and geothermal water for up to 12 months. The healing behavior was assessed using crack self-sealing index (ICS), ultrasonic pulse velocity (UPV), stiffness recovery index (ISR), chloride penetration resistance, and SEM/EDS analysis of healing products. Results show that narrow cracks (0.1 mm) healed faster and more completely than wider cracks (0.3 mm). Tap water promoted the most efficient healing, achieving 100% crack closure within three months, whereas saltwater and geothermal water required longer exposure, with partial inhibition caused by chloride and sulfate ions. Nonetheless, prolonged immersion allowed significant sealing, stiffness recovery of 20–40%, and reduced chloride penetration. SEM confirmed calcium carbonate as the main healing product, with higher crystallinity in tap water. The findings highlight UHPC’s robust self-healing ability even under harsh environments, though long-term field validation and studies on variable crack dynamics remain necessary.