See subject. See photo.
See friends, family.
You do know that’s a picture and not video, right? 
Inertia.
Water looks pretty calm. Sailors are used to standing on a deck, at sea. What’s the issue?
Well, I was going to ask the same thing about big ships too, but I thought subs are even harder because they’re narrower.
Win.
Do they (does one) feel the sway?
I’ve stood in that same position on the deck of a submarine many times (without Santa). The picture shows a 688-I submarine (it may even be my old submarine) returning to port, coming up the Thames River in Connecticut. That’s the Gold Star Bridge and the railroad bridge in the background.
On deck is the group of sailors (along with an officer or two) that assist with with mooring lines and deck cleats on the sub’s top deck. When returning to port, they stand on the deck at attention (because it looks more professional than a group of sailors shooting the bull and smoking).
Anyway, once you get into the Thames, the water is very calm. The sub is moving very slowly (<5 knots). You have a tugboat for close maneuvering. There’s very little sense of motion. The only real danger is that someone could possibly trip or walk too close to the side and end up in the water, but I never saw it happen. The top of the deck is coated in non-skid to help prevent this.
Farther out to sea, where the waves and sea state are higher, sailors generally remain on the sail (i.e. conning tower). If it’s absolutely necessary to go onto the deck, a sailor needs to have a harness that’s connected to a grooved rail that runs the length of the hull.
It was the USS Miami. Just a year before it’s “accident”.
I can remember seeing guys fall off the Spadefish down in Norfolk in a winter transit in, the Chesapeake can get a bit rough at times and NOB Norfolk is not as sheltered as Groton is.
Don’t they have gyroscopic stabilization?
There are gyros for the navigation equipment, not the submarine itself. The submarine itself is stabilized by a combination of fixed ballast, variable ballast tanks (containing variable amounts of seawater), and the main ballast tanks (along with the control planes when submerged).
They are used to standing on a deck, which looks like this. In that sub photo, it looks more like standing on a log. If the sub rolled a little it looks like you’d be dunked.
Back in the age of sail, they’d pull in like this..
Made for a good conversation-starter with the local girls
We still man the rails occasionally, it’s just not as impressive with 4 or 6 guys instead of 40.
The thing about subs is that before you can man the rails, first you have to put them up without falling overboard.
I imagine the camera angle is part of it, but that submarine looks a tad ‘small’, IMHO. Any idea about what type the one in the pic is? 
in this slightly-surfaced position, unlike with a boat, the vast majority of the sub is underwater, well below the waves on the surface. The center of gravity is likewise well below the surface. So any surface waves (and this does look like very calm water obviously in a port) won’t have nearly as much affect.
Thanks to all.
::…ATTENTION: Standby for correction on “boat”…::
Yes, and which holiday has late-Elvis visits like Christmas has Santa?