AIMS: The aim of the present study was to describe heart rate profiles and heart rate variability patterns in non-selected scuba divers of different ages under non-experimental real-world conditions. METHODS: We used specially designed silver-loaded polydimethylsiloxane dry electrodes for underwater ECG recordings. With a custom-built setup, heart rate profiles and heart rate variability patterns were documented before submersion, during diving and after resurfacing in 18 separate dives. RESULTS: Heart rates of the divers just before descent were remarkably high (median 114 bpm, interquartile range [IQR] 83–154) with a statistically significant rapid decrease after submersion (median 90 bpm, IQR 70–116; p = 0.008). The percentage heart rate reduction by submersion was individually very variable (median 21%, range 5–39%). We noted a general increase in autonomic nervous system (ANS) activity without predominance of parasympathetic parameters, suggesting a concomitant sympatheticadrenergic activation. CONCLUSIONS: Scuba diving under real-world conditions by non-selected divers is characterised by relatively high heart rates just before submersion, an individually variable but significant bradycardic dive response, and induces an immediate and sustained parallel increase of parasympathetic and sympathetic-adrenergic autonomic nervous system activity. These observations could explain several specific pathophysiological mechanisms of diving incidents (haemodynamic decompensation, arrhythmias, acute coronary syndromes) and underlines the importance of cardiovascular risk stratification in diving eligibility assessment.