This is a glitch I found in the Marina level that basically allows you to see the map in its entirety. There isn't an easter egg or anything of the sort to find, but you may find it funny or interesting nonetheless.
Here's a guide on how to exploit it:

In Marina there's a ramp near the port's entrance.



If you try to go on the water on foot the antagonist will stop himself, you'll have to use a vehicle. To do so, simply kill enough civilians, eventually the cops will come in their cars and vans (the quickest way for this to happen is to attempt the hotel side-quest).

Once you got in one, simply drive to the same ramp and you'll be able to drive over the sea with no problems. You'll also be able to drive over the port and the boats.



You can't get out of the vehicle as long as you're on the sea but you can go as far as the map's borders. Do not trespass them though, or you'll fall off and die. You can also fall off the map if you bump too hard against an obstacle, so be careful to where you drive.

That's all, try it before they patch it!