Go to You Tube and watch "two weeks in Hell"
YouTube - Two Weeks in Hell (Part 1 of 9)
And then watch
See if you think his shoulder will hold up. Watch the log drills! In part 1 look at the instructor-- he is just as skinny as I was.
There is an extreme amount of psychological testing to include darkness, claustrophobia etc. Any mental phobia that would keep you from 100% efficiency is looked for, including a imprisonment/torture phase.
The attrition rate is 60+%. Then you are in a infantry slot for the duration of your enlistment contract if you don't make it. Nothing wrong with infantry as I am as proud of that as any other dodad I have.
Two benefits is that we are really good at staying alive in extremely dangerous situations and we know very well what grade of steel our spine is made from. As a father the first benefit is important to me. As an old geezer I wish the rest of me was made of 340 steel.
Getting into dive school is dependent upon which ODA (A-team) you are assigned to and you cannot guarantee the you will be assigned to a dive ODA. My ODA was just plain Jane ordinary with no extraordinary training required.