["The Journal of Physiology, EarlyView. ", "\nAbstract figure legend Healthy, trained divers were studied before, during and after diving in open seawater with different techniques. SCUBA divers (diving to 15 or 40 m with air; cycling at depth) and breath‐hold divers (BHDs; sled‐assisted dives to 15, 25 or 40 m) underwent underwater and surface arterial blood gas (ABG) sampling. In SCUBA divers, arterial partial pressure of oxygen (PaO2) rose markedly at depth and returned to baseline on resurfacing. In BHDs, PaO2 increased at depth but fell to hypoxaemic values on resurfacing before breathing; a subset (three BHDs) did not display bottom hyperoxaemia. Lung ultrasound (US) showed post‐dive B‐lines and pleural irregularities, more pronounced after deeper dives and in breath‐hold divers. Plasma syndecan‐1 (SDC‐1) and heparan sulfate (HS) increased post‐dive, showing endothelial stress.\n\n\n\n\n\n\n\n\n\nAbstract\nUnderstanding of pulmonary gas exchange measurements in divers at sea is incomplete. In this study, arterial blood gases (ABGs) were measured in SCUBA divers breathing compressed air and pedalling at depths of 15 or 40 m in seawater (msw). In breath‐hold divers (BHDs), ABGs were obtained before, at 15, 25 or 40 msw, and at the surface before breathing. Lung ultrasound was also performed in both groups before, at 15 msw, and after all the dives. Blood syndecan‐1 (SDC‐1) and heparan sulfate (HS) were also measured. Among 10 SCUBA divers (one female; ages 32–57), PaO2 increased at depth as predicted. Among 12 BHDs (three female, ages 33–62), PaO2 rose at depth and decreased on surfacing; two participants at 15 msw and one at 25 msw did not develop bottom hyperoxaemia. Lung ultrasound was normal at 15 msw, while interstitial oedema or pleural irregularities were found after surfacing in most SCUBA divers and BHDs. In SCUBA divers, significant post‐dive increases occurred in SDC‐1 and HS; in BHDs, a significant increase was found in HS after the 15 and 25 msw dives, while SDC‐1 increased after all depths. Compared with warm‐freshwater experiments, ABG values in SCUBA divers were similar, while in BHDs relative hypoxaemia at depth was less common. Elevated levels of glycocalyx markers were consistent with endothelial stress, possibly providing a mechanism for fluid to accumulate in the pulmonary interstitium and explaining the ultrasound abnormalities.\n\n\n\n\n\n\n\n\n\nKey points\n\nThe understanding of lung–environment interactions during open‐sea diving remains limited.\nWe integrated underwater and surface arterial blood gases, lung ultrasound and endothelial glycocalyx markers (syndecan‐1, heparan sulfate) to quantify gas‐exchange perturbations and lung stress in SCUBA and breath‐hold divers (BHDs).\nSCUBA: arterial oxygen (PaO2) increased at depth and returned to baseline at the surface; BHDs: PaO2 increased at depth (except in three participants), then values fell to hypoxaemia on surfacing.\nPost‐dive lung ultrasound showed subclinical interstitial oedema – from focal B‐lines to diffuse patterns – and pleural irregularities more marked after deeper dives and in BHDs than in SCUBA.\nCirculating glycocalyx markers increased post‐dive, consistent with endothelial stress.\n\n\n"]