The number of in-water mass casualty incidents has increased in recent years and provides significant challenges to rescuers. Existing triage systems require the rescue of immersed (in water) casualties before triage is undertaken. A tool that enables triage to be undertaken before rescue, and therefore the prioritisation of that rescue, should improve the efficiency, efficacy and survival rate associated with the management of such incidents.In this paper, we describe the rationale and development of a proposed novel 'in-water mass casualty triage tool (IWMCTT)' to assist in the swift and effective triage of those in the water in mass casualty situations before they are rescued, based upon the likelihood of survival after immersion. The tool is based on a review of the literature related to the hazards associated with immersion, most notably drowning.The IWMCTT employs a sequential approach to streamline the identification and prioritisation for rescue of immersed individuals; it considers factors such as hazards, visibility constraints, purposeful swimming, moving or floating, airway position, availability of flotation assistance and flotation device effectiveness. It categorises casualties from W1 (high) to W4 (low) priority for rescue.The proposed IWMCTT offers a potential solution to some of the challenges faced during water-based mass casualty incidents; providing rescue assets (rigid-hulled, inflatable boats, ships and helicopters) with a rapid and effective approach to assess and prioritise individuals for rescue and medical attention, hopefully thereby reducing mortality and morbidity. The IWMCTT requires further evaluation and validation.