Most optics are useless in heavy rain. Even ACOGs. The ELCAN is the only scope I've seen that is still usable with water on the lenses. Yes, I know it's too big and makes a lightweight AR-15 top heavy.
I agree with everything else crazy cat said here, except for the optic in rain.
If you are talking about a magnified optic, then yes, water on the lens is an issue.
If, however, you are using a 1x holographic red dot, like an AimPoint PRO, then the water should not be a problem.
You are supposed to use both eyes when using a 1x CQB optic. I can leave both the front & rear lens covers closed on my PRO and see the red dot through the clear rear cover using my right eye. The target is seen with the left eye.
Together, both eyes give me a red dot on the target in the same spot I would see were I looking all the way through the scope.
It's one of the cool things about red dots. If you're in a hurry to shoot, you don't have to open the covers on the PRO -- just make sure to leave the power turned on (the battery lasts about 3 years if left on at the mid level brightness).
Once you add a magnifier to the red dot, however, you need to close the left eye -- that is, unless you have super powers that can magnify the left eye's image to match the scope!!

Only a 1x optic will permit you to have both eyes open, just like iron sights.
