Just like the title says, I'm on that quest and got into the ship but cannot find a way out.

Also how do you change the gamma settings? it is so dark in there I can't see anything.

i did that quest yesterday and as far as i can remember it is very easy to get out.
Did you go in at night?
And did you try to escape through a "window" instead of the "door"?
I think the ship might have an opening that looks like you can get through, but you can't.

maybe that helps.

Gamma can be changed in the video options somewhere, not hard to find.

I wound up loading a saved game in order to get out of there and then got the treasure by reaching in through a hole in the side of the ship --- WITHOUT going inside the ship.

That ship is a trap. Very difficult to get out of there it seems.

I looked all thorugh video options and could not find gamma but I know I set it once because it said to adjust the screen until you could barely see the pattern. But now it seems to be gone. Perhaps we only get once chance to set it and then they take it away.

Try setting your game to full screen mode then look again in the video options, it should pop up.

There is a ship around that area where if you jump in, it is very difficult to get out. There is some youtube videos showing you how to escape.

I got stuck in one and my previous save game was too far back, so I just ran at the ramp until I got a to a very small point where you can jump, it took around 15 minutes, haha.

That's an actual bug and oversight from the devs. I did the exact same thing, jumping in from the top, exploring the hull, then unable to exit due to the fact it won't let you climb out. Had to use an earlier save and avoid it.

Go into your nvidia settings tab by right-clicking your desktop window, select nvidia (assuming you have a nvidia card?; imagine AMD cards are similar) settings then adjust gamma, color, contrast, etc. I had to increase brightness a bit for this game as it was too dark, and I wasn't going to mess with cat potion every 10 seconds.

Use Cat (eyes) Potion

Get "white whale", this potion makes you holding your breath some more time.

To get into the ship, there is this window like opening, and from there it is only a short way to the chest and back.

So: the potion and witcher senses will make this more easy.