A couple of water drops falling somewhere in your case have a very small chance of frying the thing. A drop would have to precisely hit a live pin somewhere, I think chances of that are slim.
But yeah, as was asked before, I wonder how often you would use your pc with fully wet hands. Maybe dry your hands on a towel after washing them.
That seems like a pretty extreme measure for protection and I have so many questions... How often are you using wet hands with your mouse? Are the vents directly below the edge of the desk/table so that any liquid would drop off directly into the vents? Could you simply push the desktop back a little and not have the vents directly below the edge where liquids might fall? Do you have a cloth mousepad that would likely pick up any small amount of liquid that might come off your hand?
How often is kinda like it depends. But I mean, it can happen, my hands aren't always perfectly dry, I might even forget to dry them properly. I try to be careful now, but it can happen.
So, like 10-20% of the case is not under, I can't push it entirely back, but I could push it back some more (and even then, drops of water don't always fall perfectly vertically, so movement of hand can throw them under the desk anyway).
No cloth mousepad, I don't even use a mousepad.
Are you using top fans? If not, cover it so that the air is simply sucked in the front and pushed out the back.
I am not using any top fans, no, only front and back (and bottom if we include PSU fan). Thought the vent on top could also help with airflow though, not sure.
You want airflow to be like a wind tunnel. Open vents without fans hurt cooling, not help it.