I'm still testing this in game but here is what I've got so far. The only side effect is that at about 50 meters you can see a faint light through the water that is the horizon. For some reason it comes through the underwater fog a bit. But how often are you looking out the periscope at that depth anyway?
If anyone is interested I can release it as a mod. I find that it looks even better when installed on top of Sh5EnvGold, the original environment mod for SH5.
This picture is underwater at high noon on a clear day.
