A little water *on* the air filter won't kill it. Water in the fuel might, as it's passed through the injector, but atomised water being passed through the intake valves will not harm the motor.
If it takes a gulp - meaning more than just a mist - and the engine tries to compress the water, it'll probably snap a con rod or blow the head gasket, crack the head etc. You'd know pretty quickly because the engine stops dead until you repair the damage.
I wouldn't worry. I might replace the air filter and give some thought to a snorkel.