Water falling on top of central AC
The outdoor units are meant to have water hit them.  Only a flood or major major standing water would hurt the unit - unless the water came down at such a high rate that it bent something.  

Go outside and look at your unit.  It will have a "section" or box with the electrical compontnents.  It would basically have to flood to that height to hurt your unit.  

Water could have a good impact in that it would cool your unit and clean off dirt and small debris.  I can't really see how this would hurt anything.