Both normal fog and the new altitude-based fog affects the underwater environment. Regular fog makes the water murky with a shorter view distance as it does above sea level.
The new fog is subtle and can remain unnoticed until you ramp up the last value.
I wouldn't imagine I'm the first to discover this, but I have searched all the entries that come up with "Fog" as the search filter and apologise if this is a duplicate. {F19637}
Altitude-based fog:
The view distance closes to zero (see screen in attached zip). When wading on the surface the fog underneath the surface is the same colour/density as that above and creates a grey-on-grey effect with the surface water inbetween which makes it look like a graphical glitch (also see screen in zip).
Normal fog:
Visibility will be normal underwater
Visibility will be considerably shorter
If this can be controlled with values for above and below sea level then this could be useful for users to adjust underwater visibility.
The graphical glitch-ish issue is a problem, but the low visibility on the second one is probably due to less sunlight getting to the water which results in smaller visibility.
/upvoted
Yeah, I tended to try to find the positive in that there's less light, but it would really need to be a darker shade rather than the same grey as the fog above the waterline to make for at least a reasonable compromise. Different fog values above and below sea level would be good to create murky water or reduce the view distance to more realistic levels (should one chose to have it in their created content), but it may not be possible due to the rolling waves of the water. But it's not for to a fool like me to judge what's possible, just to suggest.