Introduction: This field-based case study assessed linear accelerations of the torso centre of mass (CoM) and the tibia using two wearable sensors in a triathlete performing 20 minutes of continuous deep water running. Methods: One triathlete (36 years; height; 151 cm; weight; 63 kg) participated in this field-based case study. One wearable sensor (an accelerometer) was located on the lumbar five/sacrum one (L5/S1) spinous process as a proxy for the centre of mass (CoM) and on the midpoint of the right tibia to capture linear acceleration magnitudes. The participant then performed 20 minutes of deep water running that was completed at a self-selected pace. Results: No significant differences (p<0.05) in torso CoM and tibia acceleration magnitudes were detected in 20 minutes of deep water running. Despite this, the magnitude of torso CoM and tibia acceleration, notably in the anteroposterior and vertical directions, increased at the midpoint of the deep water run, possibly indicating the onset of fatigue. Conclusions: This field-based case study suggests that a wearable sensor can reliably detect variations to the torso CoM and tibia by way of linear acceleration magnitudes in 20 minutes of continuous deep water running. Based on these results, it is possible that the increased acceleration observed at the tibia were due to the onset of fatigue. Future investigations with a larger number of participants are needed to further explore the relationship between torso and tibia acceleration and fatigue in deep water running.