Ever since Fallout 4's release, inquisitive players have been obsessed with one thing: what in the world is Fallout 4 hiding? Take a look at a map of Fallout 4:
[Source: Fallout 4 Wiki]
There is a lot of water in Fallout 4. Surely there must be something under there? At least, this is an assumption players have been making for over a month now. To wit, if you type 'Fallout 4 underwater', Google will likely autocomplete with "secrets". The mystery is a Thing within the Fallout fandom.
Consider the following. Fallout 4 has a perk called "Aqua Boy", which lets you breathe underwater without taking any damage. We also know that at one point, Bethesda was developing a harpoon weapon for the game. It even has a special bubble animation that percolates from the gun whenever it is used:
With all of this 'evidence', it's not surprising that some players began exploring the depths of Fallout 4 to see what could be found. One player in particular, Lavon Smith, has spent 30+ hours going through Fallout 4's waters. He says he's traversed nearly 100 per cent of the water in Fallout 4, all of which he documented in a huge imgur you can find here. Some highlights:
A man in a wheelchair in the middle of nowhere.
Boats. Of course.
Skeletons AND teddy bears.
This poor guy didn't make it.
Random, empty pipes.
Surprisingly normal-looking fish.
Not-so-normal-looking-fish. This one is a Jaws easter egg, actually.
Yes, there are apparently enemies floating in the water, too! Legendary enemies, too.
A boat with a hilarious number of cat paintings. MEOW.
MYSTERIOUS STRUCTURES. Some of them even have symbols/words on em!
A suit of Power Armour. Well I'll be.
In short: there is no huge, mind-blowing secret hiding in Fallout 4's waters, as best as anyone can tell. That's OK, though. There's still plenting of cool stuff hiding under there, as well as some loot.
Helpfully, if you'd like to explore this stuff on your own, Lavon Smith provided a map that highlights areas of interest:
And you should definitely check the original imagur, which provides all sorts of other images of what's hiding in Fallout 4's water.
Of course, just because people haven't found anything big within the waters of Fallout 4 doesn't rule out the possibility that Bethesda might go there with the DLC. One can dream, at least!