Information on vertical space use in air-breathing aquatic animals is crucial for understanding their foraging ecology and for informing conservation policies. Fully aquatic viviparous sea snakes are the most speciose group of marine reptiles, yet logistical difficulties have precluded fine-scale and long-term tracking of their diving behavior. To gain insights on diving behavior of ornate reef sea snakes (Hydrophis ornatus), we surgically implanted depth-temperature loggers (weight in air 2.9 g) into free-ranging snakes in Okinawa Island, Japan. Through scuba diving, we recaptured four tagged snakes (105–230 g body weight) and obtained data on 23,443 dives across 455 days. Maximum depth and maximum duration of dives were 129 m and 5.4 h, respectively. The maximum dive duration observed in this study greatly exceeds the previous field record for sea snakes (3.6 h). Individuals spent the majority (<95%) of their time at depths shallower than 40 m. During the day, post-dive surface intervals were typically less than 10 s, and snakes avoided very shallow dives (<5 m), whereas at night they occasionally remained at the surface for up to 90 min. This diel contrast in diving behaviour likely reflects avoidance of surface-associated predation risk during the day and surface resting at night. GLMM analyses showed that, after controlling for maximum dive depth, water temperature had a negative effect on dive duration. Maximum dive depth had a positive effect on dive duration, although the longest dives were not the deepest. This study advances our understanding of diving physiology and behavioural ecology in sea snakes and provides a critical baseline for predicting behavioural responses under ongoing climate change.