It must be The Hunt for Red October. I think I remember this scene where Jack Ryan jumps over the octopus to do hand-to-hand battle with pirates.

Captain Goodnight

Another hint: The game is available on GOG.

Ultima 2 comes to my mind but I guess I'm focusing too much on the Soviets. Although it DOES also have pirates and some sort of seamonsters.

Aquanox?

Nope and nope. A third hint: Think about the guy freaking out on top and what situation he's in. As said, this is in no way depicting the actual game. The only concrete thing I'll let slip is that the sea plays an important role.

Myst?

This is starting to get silly.
Just like Camelot.
and I sure don't want to go to Camelot.
It is a silly place.

Nope. Maybe I made it a bit too abstract. Hm.

Neighbours from Hell maybe? :p

Oh nevermind, you said the sea was actually literal. Hm, hm.

I like abstract. I was only joking before, and I get the feeling a lot of people are going to feel really stupid when the answer comes to light.

Wait wait... is it possible that the guy is not falling, but rising?

The only GOG game remotely reminiscent to the picture that I played is Fallout 2.

But this can't be it, correct?

Huh, okay, out of desperation I will go with these two games:

Sanitarium
Personal Nightmare

Maybe your pic is supposed to depict the insanity of Sanitarium or YOUR very own personal nightmare (maybe you're a child of the 50's still living in fear of a soviet invasion and mutated squids which resulted from a nuclear holocaust). :P Sadly the sea doesn't really fit into the picture. -.-

I see. Hostile Waters: Antaeus Rising still kind of fits.