Open water and marathon swimming is the other format where altitude training matters disproportionately for swimmers.
Some major open water events sit at altitude. Lake Tahoe (1,897m / 6,224ft), the Lake Zurich crossing (406m / 1,332ft, low altitude), Aconcagua marathon swim (in altitude lakes), and various high-altitude crossings in the Andes and Himalayas. For events at altitude, pre-race exposure is preparation rather than optimisation. Athletes who arrive partially acclimatised hold pace through the back half of the event. Athletes who arrive sea-level acclimatised lose minutes.
Most open water altitude protocols use 3 to 6 week blocks at 2,500 to 3,000m (8,202 to 9,842ft) timed 1 to 3 weeks before race day, with the higher set point reflecting the goal of acclimatisation to race-day altitude alongside Hbmass adaptation.