For best results, use shampoo and then conditioner.
Otherwise just use shampoo. Your hair will probably suffer, but it'll still be clean.
If you ask me, doing it any other way means you're an idiot. Especially if you use JUST water. That doesn't get anything actually clean.
Getting away from HOW to wash your hair;
Of course, having dreadlocks and some other hairstyles means that you absolutely must make sure not to get your hair wet to any significant degree, so there are a few exceptions to which not washing your hair is acceptable/required.
Also, you hardly have any hair to wash if you're bald now, do you?