On 09 December 1999, a USMC Force Reconnaissance platoon was assigned to perform a VBSS on the USNS Pecos as part of a training exercise in preparation for deployment with the 15th MEU(SOC) in January 2000. The assault team was comprised, in part, of two CH-46 helicopters assigned to the Marine Medium Helicopter Squadron 166. These were designated numbers 1 and 2 , with the platoon split between both birds. They were going to land if there was room, or fast rope if not. A SEAL platoon was assigned to assault simultaneously from surface/up. US Navy Special Boat Unit rigid hull inflatables (RIB) and their crews were on other side of ship, carrying the SEALs who were to perform the assault. The pilot of helo #1 struck, or became fouled in the netting surrounding the stern of the ship. The A/C immediately became unstable and rolled into the ocean. The A/C sank and #2 waved off. The Force Recon personnel were standing up and preparing to "Fast Rope" on to the deck at the time of the accident. Eleven survivors were immediately pulled from the water, however six Marines and one sailor died in the crash. The helicopter went down about 1:16 p.m. PST (2116 GMT) about 15 miles (24 kms) west-southwest of Point Loma. All Force Recon personnel were from the 5th Platoon and part of the 15th Marine Expeditionary Unit based at Camp Pendleton, north of San Diego

Думаю именно тогда и включили в состав комплектов SEA MKII (дыхательный аппарат на 60 вдохов).