There is a submarine in fallout 4. Yes, a sub. Yes, you can go into it. Yes, there is a quest that goes with it.

How do I explain this.... First, go straight into the ocean from the Boston airport, looking at the map, go a little west. Look for a group of boats floating in the middle of the ocean, 4 of them, to be exact. Head there. There should be a pole (or whatever you would call it) in the middle of it. go there. Now you at the submarine. Just for reference it is kinda in the middle of general atomically factory and the Boston airport.
If you are to post this anywhere else, please reference my username, thanks!

If you need any help finding it, or on the quest, reply to me

I haven't done much underwater exploration yet. I'll definitely be going to have a look for this though. Thanks for the heads-up!

the submarine is hardly a big secret. exploring the docks will introduce you to a kid who tells you something is in the water. You can even see the periscope from shore.

And you can even explain "what happened with the monster" (ie lie) to the kid afterwards.

yeah I told him it swallowed me up haha

Yep, and if you talk to the boy at one of the boathouses, he starts the Sea Monster quest.

I actually felt for the captain, so I helped. I haven't used any of his rewards yet, but in time... stuff will light up the night sky.

I have only used one of the captain's "gifts" so far,

It was when I was at a certain area near Salem, and did not feel like fighting all of those raiders to get inside . . . .

WORTH IT ! ! ! !

I helped the captain then tried those "gifts"... thought they were lame. Went to another save, helped him to get the experience points... then looted his corpse of his cool hat and clothes, then tossed those gifts into the ocean.

Did swimming out to the "monster" give you a sudden case of "clench the bowels" syndrome or was it just me. I was swimming out there, and I kept imagining this giant, horrible monster rising out of the depths.

thats kinda neat, a quick youtube search gave me its exact location tho, rather than just a vague "over there" description

https://www.youtube.com/watch?v=S4wio20BPzo

During my second attack on the BoS Airport, I used the Minutemen's artillery and the submarine's tactical warheads on the forces there.

The BOS is very-very confused about the Institute having nukes and artillery.