Hello Rosegoldslugs,
Thanks a lot, you solved my problem in two different ways !
First, the “compute fog per pixel”, which i didn’t know, did the trick.
Anyone getting this problem, the first solution is here:
A very nice news is that this doesn’t seem to affect the perfs at all (at least in my setup), in fact, i ran a test with a large amount of water and couldn’t see any change in the displayed fps.
Next, yes, i have both atmospheric and exponential fogs.
As a test, i turned off the fog/pixel option, then turned off the atm fog (hidden in game = true) and again, it worked well.
So second solution anyone: don’t use atm and exp fogs together.
I need both fogs so i’m going to stick with the first solution (fog/pixel), especially as it doesn’t bring any perf issue in my game.
So a huge thank for your answer 
Now to answer your questions, in case it’s relevant for anyone willing to understand what’s going on:
My levels are generated at runtime and can have height variations so i can’t use a single large plane for water. Instead my water is composed of multiple small squares.
Here is the same scene without and with the green surrounding. You can see that without the surroundings the water is just a set of square cells.
The lighting mode is “surface transluency volume” (as can be seen in the hereabove capture, where i have only changed the “fog/pixel” option).
Thanks again
Cheers
Cedric