Waterproof clothing is essential equipment for a marshal. You may be on post all day in pouring rain with little or no shelter, staying warm and dry is of utmost importance for health and sanity.
A waterproof jacket that is large enough to fit over layers of warm clothing and a pair of waterproof trousers that can be pulled on over boots and overalls are recommended. As with all clothing worn by marshals on circuit these should be of a colour that does not clash with flags (no red coats!), hi-vis orange is most common.