Of course there is that Thai law making wearing underwear mandatory when leaving your home.
White x-ray clothing refers to white cotton garments. They have a tendency to become 'see through' when wet. Unless you are an exhibitionist, standing naked in public can be mighty embarrassing and can result in some sort of police intervention.
With all this talk of water shortages, expect some official guidelines / restrictions to the water throwing.
A nationwide ban is unlikely as the event is a major money maker for tax coffers.
It's also a period of increased alcohol consumption and police checkpoints