I saw them train when I was a the NavAmphib Base at Coronado in 1968. Just the UDT portion of the course is murder. I was never tempted. They are all better men than I am and my hat is off to them.
Yes, in late July-August 67 Navy UDT types who were at Little Creek, VA were in my airborne course at Fort Benning. Darn those men could run. At the end of tower week, we had a 2 mile plus or so run. I lost out to a Navy officer. The truth is all Special Ops types have a purpose. At the start of the Special Forces Course for both officers and enlisted we had a navigation test. That test was mostly in a swamp plus other difficult terrain to navigate. I took about 4 hours to finish. Some were still trying after 8 hours. If no pass-No SF. These training areas serve a purpose.
I cannot even imagine it.