You're kind of fucked in that case regardless, and waterproof jacket will be the least of your concerns. All you have is a small handgun.
Pilots spend most of their time flying over either friendly territory or open water. Assuming they survive the crash intact, cold is overall their #1 threat.
Wanted to add that a bunch of planes from back then didn't have sealed, heated cockpits like we know today.
The vast majority of military cockpits aren't pressurized even today.
Yes, they are (F-16 most commont)[1] (F-16 pressurized)[2]. It's just not enough to avoid needing additional air supply to prevent hypoxia.