This paper employs an anomaly detection algorithm to assess the normal operation of underwater gliders in unpredictable ocean environments. Real-time alerts can be provided to glider pilots upon detecting any anomalies, enabling them to assume control of the glider and prevent further harm. The detection algorithm is applied to abundant data sets collected in real glider deployments led by the Skidaway Institute of Oceanography (SkIO) and the University of South Florida (USF). Regarding generality, the experimental evaluation is composed of both offline and online detection modes. The offline detection utilizes full post-recovery data sets, which carries high-resolution information, to present detailed analysis of the anomaly and compare it with pilot logs. The online detection focuses on the real-time subsets of data transmitted from the glider at the surfacing events. While the real-time data may not contain as much rich information as the post-recovery data, the online detection is of great importance as it allows glider pilots to monitor potential abnormal conditions in real time.