Heavy fog.

@scaryskeleton24
We recently spoke of this in-game, perhaps two days ago.
Although yes, it would be great. it would also put an enormous strain on the graphics. More so on low-end PC systems like mine that barely scrape through the minimum requirements of the game. With an average of only 15 fps I could imagine fog taking that into the single digits.

@shaggy2000 I can appreciate what you're saying but I don't believe it works that way. Even though the islands wouldn't be visible on screen, they will still be drawn by the graphics memory.
Although I haven't yet made maps for the U4 engine based games, it's a general concept for particles (ie. dust, smoke, fog, etc.) involving the VisNumBytes and VisData size. This refers to the number of bytes the graphics card is required to load in any given area of the map.

Map designers could possibly combat this by not including the fog for low-end video spec settings but then you are left with a HUGE advantage to those of us running graphics in the lowest video settings. This has been the case for the past 15+ years since Metal Of Honor Allied Assault (released in 2002).
People with high-end systems ran full "effects" in the highest video settings but smart people ran it without effects such as dust, smoke and fog to see everything clearly.

Although it would make a pleasant change for us low-end systems to have some sort of advantage, it wouldn't stop those with high-end systems from doing the same by deliberately turning down the graphics.
I don't think Xbox would have that option so it would create even more whining from the majority of the SoT player-base (Xbox = 86%) and in this case, rightly so. I would have to agree with them that the disadvantage then, on graphics alone, would be game breaking.

If I can see you but you can't see me simply because your view is obscured by fog and mine isn't.
Just imagine how many more false accusations there will be about esp and aim bots then. lol

I don't mean to discredit the idea of fog, it would be awesomely eerie on still water but as the game was sold as playable with 1GB VRAM they can't change that now so it really doesn't leave them with any options.
To be honest I'm concerned about how they are going to handle active volcanoes with all that smoke, etc. The teaser look very hazy. Nice effect but hell for graphics memory.

@shaggy2000 Fog/smoke/mist is one of the hardest things to render is why. Also, the islands aren't derendered because they are covered by fog, so the strain is still there and then you're just asking your CPU/GPU to work harder because of some mist/smoke.

@guepard4 In consideration we don't know exactly how the game is built/what is taxing vs what isn't in their current build. It's easier to say it wouldn't or would be. I am simply saying as a whole, mist/fog is something people generally complain about as resource taxing in the past. When you combine that with ships, physics, players, items etc all moving around at the same time, you're bound to see some performance impact. Not quite sure the XoneX could hold up honestly.