Submariner hat

The  is a piece of armor in Fallout 4.

Characteristics
It provides a Damage Resistance of 0 as well as a bonus of 5 radiation protection and 2 Endurance.

Locations
Worn by Zao on the submarine Yangtze. You can pickpocket it off him (with the appropriate perk). During the 2nd part of the quest on the submarine, you'll encounter the First Mate, who is also wearing one.