You really dont want to use water. There is a special product used for flushing a/c coils.
Consider this... When the gas leaks it takes some oil out with it. Thus an accumulation of dust and dirt apears. Most leaks are in the high pressure side of the system , i.e. condesor, compressor, hoses, and even more often in the fittings.
Bottom line, If that vaporator coil is clean and dry its probably fine.
If it has a oily nasty area, replace it.
