The endoscopic resection of gastrointestinal tract lesions embraces different types of techniques, ranging from conventional polypectomy/endoscopic mucosal resection (EMR) to the field of third-space endoscopy, including endoscopic submucosal dissection (ESD), full-thickness resection and peroral endoscopic myotomy (POEM). Parallelly, the advent of underwater techniques has served as an add-on for both basic and advanced procedures, since its first report in 2012. We aimed to provide a comprehensive update on the state of the art about the feasibility of underwater basic and advanced techniques for GI endoscopy. Underwater EMR (U-EMR) has proved effective and safe in treating > 10 mm sessile or flat or all-size recurrent colonic lesions. Conversely, although data show good effectiveness and safety for <10 mm lesions, it is preferred when high-grade dysplasia is suspected, favouring cold snare polypectomy for all other cases. Moreover, promising data are emerging regarding the feasibility of U-ESD for difficult-to-resect colonic lesions. U-EMR represents a standard of care for treating < 25 mm superficial non-ampullary duodenal epithelial tumours. Data regarding oesophageal, gastric and ampullary lesions remains limited to small cohorts. Finally, using water immersion for POEM has shown a reduction in procedure time compared to the CO insufflation technique for vessel coagulation, albeit in a single-centre experience. Based on these results, U-EMR has become a standard for treating intermediate-size colonic and non-ampullary duodenal lesions, as highlighted also in the European Society of Gastrointestinal Endoscopy guidelines. Promising results have been shown in third-space endoscopy studies, even though further prospective studies are awaited to standardise the technique for both ESD and POEM.