I'm not sure on the casualty rate of US subs overall, but German U-boat had a 75% casualty rate. https://historyincharts.com/german-u-boat-losses-in-world-war-ii/. I believe that--beyond small units which were sometimes wiped out--this was the highest casualty rate of any broad classification of service in WW II.
Yeah, I knew being sent out on a U-Boat was close to a death sentence.
The original context was from the Allied or American perspective, though.
For that, I still think a bomber crew in Europe was about as bad as it could get.
If you expand it to all combatants, though... there's a reason Das Boot is the film it is.