According to Frank King in the Star Magazina, MB permitted the use of drinking water but you can use distilled water if it's mixed with anti-freeze. It's then no longer distilled water.

The problem with *drinking water* is that it's generally tap water and you never know what's in it. I never permit tap water in my cars except that it's too late now because the dealer used it to flush out the cooling system when he replaced the head gasket in my car.

I've heard that distilled water can cause the head gasket problem or it could be caused by Redline Water Wetter or just a design flaw in the original head gasket. I think it's the latter.