Navy SEAL Delivery Vehicles are mini submarines with no dry interior, and the crew rides fully submerged in seawater for the entire trip. The vehicle is built to flood, so the pilot, co-pilot and four SEALs sit upright in a cramped space, breathing through the vehicle's air supply or their own gear. Rebreathers help keep bubbles from giving the team away. Reported endurance is roughly eight to twelve hours, and one account describes a 36-nautical-mile trip taking about nine hours. For the SEALs aboard, that means hours of darkness, tight quarters and cold water, with nowhere to go until the mission is done.

The cold is the real enemy, because the body keeps losing heat for hours, even in water that isn't freezing. A RAND study on SEAL transport warned that long, cold transits can hurt both physical and mental performance, and that crews sometimes have to warm up on the beach before they can start the job. That makes the latest reports even more striking, because the Financial Times reported that SEAL divers spent four months clearing Iranian mines from the Strait of Hormuz, mostly at night. Those reports describe inflatable-hulled boats and unmanned underwater craft, not SDVs, so any SDV role remains unconfirmed, and the four months covers the whole operation rather than nonstop time in the water. Either way, the next time the gym pool feels a little chilly, remember what these divers reportedly put up with.