The safe way would be to clean the valve cover by hand.
If you do use water, cover the valve cover with something to keep the water away.
These engines are prone to water shorting the plug ends because the cover for the coils/wires/plugs is not water tight and the plugs are at the very bottom of the head casing. So any water winds up right at the plug cavity. And it stays there for some time.
You may also want to change the 3 coil to plug resistor boot
ends under each coil as they are known to deteriorate from heat
fatique and cause a rough idle and plug mis-fire.
The plug ends on the other 3 plugs don't seem to suffer from this as they breath better not being trapped by the coils.
They are about $8 ea. and easy to replace...