Rumors are true: there really is a Chinese submarine in Fallout 4 and here's how you can find it in the game. You first heard about it in a military research facility out in the Commonwealth. Ok, if you want it, get a Hazmat suit and some Mirelurk Cakes, and head out to the coast.
North of the Castle and near the Four Leaf Fishpacking Plant you will find a pier that serves as home to a boy called Donny. Talk to him and he will tell you there's a sea monster, asking you to investigate. This is the mission Here There Be Monsters starting.
Swim in the water and you will find the sub in order to continue. Once you are in the submarine you will meet its inhabitants, who will talk you about their mission: relaunch the Yangtze and head back to China. Your reward is a set of 3 homing beacons that you can use in combat to launch a tactical mission attack from the departing Yangtze
Another Chinese submarine was spotted in Fallout 4, specifically in the DLC Point Lookout; it was called SSN-37-1A. Another one was mentioned in Fallout 2, in San Francisco: the Shi.
