How to clean iMac screen?
I have done this safely before with windex.  Just spray a little on a microfiber or even a soft towel and wipe down the screen.  As long as you do not drown it you will be fine.

I have taken the glass off the iMac before to work inside it and they do a good job sealing the insides from the out.

In general, when cleaning a computer, distilled water is fine as it is non-conductive.