Any car/engine that is driven in the heavy rain is going to get a little bit of water in it. It's a safe bet that during extreme rainy conditions when you can barely see and you're almost hydroplaning and there's just water every-frikin-where that your engine is pulling in some pretty wet air because while it's raining like that the humidity is usually at around 100% and all that splashing is bound to get a little spray up near the intake.
I think the fine mist metioned above is actually a naturally occoring event. Hydrolock on the other hand is where the intake literally becomes a giant straw pulling in water a cup or a pint at a time. It probably takes about that much to hydrolock an engine. Not an eighth of a teaspoon.
I am in no way responsible for engine damage resulting of someone putting water in it.