add option to disable/weaken underwater effect

[BonneCW]

[kirides]

Easiest would be to reduce the blur scale, though you will have a lower resolution render then. (jagged edges while underwater)
One could add a extended-menu option of type float 0..1 to control the scale of the effect. 1.0f is the default and way too strong for some. 0.1f is a very light blur but you see jagged edges. Most people will probably be fine with something around 0.1-0.25.