Another question - what is the water? What should I fix it?
The water you see, with black running underneath, is something I've never been able to fix. It is caused by having rivers running rather steeply downhill. They may very well be in their correct location on the game map, an follow the correct course and fall and all that, but sometimes you can 'see underneath' water that is more or less above your aircraft's altitude.
In your screencap you are flying lower than parts of the river in the background and therefore you can 'look up' from underneath the river bedding.
It's a bit of disappointment, but since the game was made to have nicely level flowing rivers at 0m altitude, I don't know how to fix it. It's one of those little flaws that have to be accepted as they are.
By the way, you'll probably notice that as you gain altitude, you'll start to see the river appear again as normal.