Sailing is done in almost all weather conditions. I don't know about your local waters, but here in Friesland half of the time it's wet. As a judge or as a race officer I need something to write on which can withstand those raindrops. For recording down a mark rounding or sketching a situation between boats or just jotting down the conditions of a particular race.
I've used different kinds of notebooks, but lately I'm sticking to the WATERBOOK MEMOPAD. You can order them in different sizes and bindings on the internet from VIP WeatherWriter, a UK based company. Small enough (A6) to fit into any pocket. Usable with most pens, but great with a pencil.