Ventile was THE material to use -and still is- for that original application.Thanks for explaining mate, appreciated
Mostly sitting, some occasional heavy perspiration from stress, and probably overdressed to deal with the subsequent unavoidable immersion, so they needed something that breathes well and which seals up on immersion. In addition, it's relatively comfortable compared to rubber suits, it doesn't melt or catch fire easily, and -in two layers and with a heavier gauge- it's pretty rugged.
I am not sure they still use them, but the RAF was definitely employing Ventile dry-suits for aircrew up to 20 years ago.