When you swim in the sea or even when it's raining it should come up with a water effect on the screen (eg. swimming up the the surface when you were underwater). Arma 3 already has dirt effects and blood effects... so why not Rain/Water Effects?
The rain or water should drop on the screen and continue to drip down like it does in other games like Battlefield 4 or Call of Duty: Ghosts.
I think it should be on the camera mode, like third person. I don't want to see dirt and rain effect in the first person view.
Only if you are wearing a divers mask or glasses, and then it should be blurred due to it being in close proximity to the players eyes which would be focusing through the water into the distance.
Human eyes do not get water droplets on them.
I attempted to provide something similar in my glasses mod http://forums.bistudio.com/showthread.php?163658-ZAM-Glasses however if it is provided engine side that's always better.