It's a Racing Rules of Sailing issue:
43 COMPETITOR CLOTHING AND EQUIPMENT
43.1 (a) Competitors shall not wear or carry clothing or equipment for
the purpose of increasing their weight.
But that still begs the question of "why".
To avoid a very strong skinny guy that can sail with minimal weight in the light breeze and then tie on 50 pounds of water to his shoulders when it's breezy. To avoid the health (back and neck) risks associated with sailing this way all day? Reduction of mobility? The risk that someone might actually wear something heavier than water (i.e. they sink)?
